The scale length of a guitar is the distance between the nut and the bridge measured in either inches or millimeters. The reason that scale length is important is that the placement of the frets on a guitar's neck is a formula that is partially determined BY THE SCALE LENGTH. In other words - necks from guitars with different scale lengths will have different distances between the frets. Fret placement is not fixed - it is variable and depends on scale length. The formula is rather complicated - thankfully there is software which will do the heavy calculations for you - you simply need to tell it the scale length and number of frets desired.
Scale length is important to understand when starting out to build your own guitar - from parts that someone else has made or from parts you're going to make yourself... because in understanding how the scale length affects fret placement, bridge placement, and pickup placement - you can avoid a lot of heartache and hair pulling (either of yourself or of those around you).
BELOW - a diagram showing some of the different scale lengths used by guitar makers. Note that the differences in neck pocket size is really common sense. If a neck won't fit the body you have - it's something that's going to take careful work on - meaning you can shave down necks or route (open) the neck pocket of a guitar body to accept a wider neck or a neck with a different heel - but scale length is the tricky "gotcha" thing that can sink the best laid plans if you are not paying attention.
ONE VERY IMPORTANT THING TO KNOW ABOUT SCALE LENGTH -
Let's say you have a Fender Stratocaster body - which happens to be the most commonly modified guitar model - and you decide you like the feel of an old Fender Mustang or Jaguar neck - you can mount a Jaguar/Mustang neck into a Strat body pocket - but the neck is set up for a 24" inch scale - and the Strat neck pocket is designed for a neck (with a fret formula/spacing) for 25.5" - what happens next is quite annoying if you don't understand the scale length factor - this new hybrid Strat - Jaguar/Mustang guitar will never play in tune. You can tune the open strings. But as you play notes on the neck - the further down the neck you go away from the nut and towards the bridge - the more out of tune each note is. This is because the frets are literally in the wrong place.
THE BASICS OF SCALE LENGTH -
Frets for a neck that is designed for a certain scale length - let's use the most common - Fender's 25.5" Scale Length for the Telecaster and Stratocaster - any neck from any builder designed for a 25.5" scale if sat side-by-side - the frets will align and be in the same place. This of course doesn't mean that those different necks will fit your guitar's body - but if they did - you'd have a guitar that would tune and you could play. Above - a Kramer Pacer neck and a Fender Stratocaster neck - both set for 25.5" scale - when you align their nuts (wow that sounded wrong) together - you'll see that each fret as you move up the fingerboard is in line with the frets from the other neck. If these two necks have the same sized heel and the same shaped heel - they could be swapped with each other. This is NOT true of necks from guitars with different scale lengths.
THE 12th FRET - GROUND ZERO - THE CENTER - OF ANY SCALE LENGTH -
There are 12 notes in the chromatic music scale. If the open note on a guitar string is "E" - then as you move up the neck (away from the nut towards the bridge) when you reach the 12th fret you are back at "E" again - just one octave higher.
Most modern electric guitar necks have either 21, 22, or 24 frets. Necks with 24 frets are "2 octave necks" because the final fret as you leave the neck towards the bridge is the same note as the open string - two octaves higher. MOST guitars are 21 or 22 frets. ABOVE - the Kramer neck (top) is a 25.5" scale 24 fret neck. The neck below it - the Stratocaster neck - is a 25.5" scale 22 fret neck. It is a little over 1 full inch shorter. THIS IS THE SECRET "OH CRAP" MOMENT MANY WOULD BE GUITAR BUILDERS FACE WHEN THEY BUY SUCH A NECK AS THE FENDER STRAT HEEL KRAMER NECK ABOVE. Sure it will fit perfectly into the neck pocket of ANY Stratocaster with a 4 bolt standard Strat neck pocket. But it is too long for a standard Strat pocket. Let me explain....
Measuring from the center of the 12th fret to either the bridge saddles or the nut should always give you exactly 1/2 of the scale length. The 12th fret is the exact middle of the scale length - or it should always be. On a 25.5" scale guitar there should be 12.75" inches between the bridge saddles and the 12th fret and the 12th fret and the nut. The 24 fret Kramer neck is perfect for that part of the equation. It is the proper distance and is in line with the 22 fret Strat neck.
From the bridge to the 12th fret is where we run into an issue. If you seat each neck in the body's neck pocket and you measure - you discover a serious problem with the Kramer neck.
ABOVE - Starting at the bridge saddles (which BTW are designed to move in very small amounts "fine tuning" the scale length for each string - because of the differences in string gauges each string needs to be a little longer than 25.5" in scale length for optimal tuning - called INTONATION). Using the saddles that are set the farthest FORWARD in the bridge - measure to the 12th fret.
BELOW - When measuring the Strat neck you get to the needed 12.75" exactly at the 12th fret.
BELOW - BUT SEE WHAT HAPPENS WHEN YOU SEAT THE KRAMER NECK AND MEASURE - The neck is off by 1 and 1/16th of an inch. This would effectively seat the neck too far out from the body and once strung and the open strings were tuned it would never be in tune as you move through the notes.
The fact is that you can add more frets to a neck of a guitar with any fixed scale - you simply need to calculate seating the neck DEEPER into the body. This becomes a problem for many guitar designs because the neck pickup placement is such that a neck with more frets would need to use the space where the pickups typically are placed. This means that special care has to be given to the guitar when it is first being designed and built to accommodate for the depth of the neck pocket, pickup placement, and total number of frets. It is very uncommon to take a guitar that was built for 21 or 22 frets and convert it to 24 and/or visa-versa.
KNOW THIS GOING INTO YOUR BUILD PLAN - DO YOU WANT A 21, 22, or 24 FRET NECK? Knowing will help you pick the right body and neck combinations.
YOU CAN modify a guitar with 21 or 22 frets to 24 - but it takes professional wood working equipment and precise measuring - it's not for the novice - you'd need to hire a qualified luthier (someone who specialized in building and/or modifying stringed instruments) to have the work done.
It's simply best if you do a little research and make up your mind BEFORE you start making lists of things you want/need your dream guitar to possess.