A stranded preposition is considered by linguists to be the preposition at the end of the following sentences:
-That's the road I walked across.
- Who did you yell to?
- It's him I'm thinking of.
Why do some people consider these sentences wrong? Well, for the most part, they don't: all but the strictest of language users (i.e. those who haven't picked up a style guide or dictionary) still consider this construction to be incorrect. Still, you will meet some people that believe this is incorrect believe it based on a fake rule invented by John Dryden based on a silly analogy with Latin (where the equivalent to a preposition is attached to the noun and cannot be separated from it) in an effort to show that Ben Jonson was an inferior poet. From there, the rule spread and caused difficulties for many future generations.
But don't let that stop you: there is nothing wrong with any of those sentences listed above! Sure, it often sounds better, especially in a formal setting, to do what is called "pied-piping" and rearrange the end preposition to another place in the sentence, but this is mostly because the sentence ends on a weak note if it ends in a preposition.
By the same principle, a preposition should be stranded at the end of a sentence when it contributes a crucial bit of information, as in "music to read by", "something to guard against", or when it pins down the meaning of an idiom, as in "It's nothing to sneeze at."
So, get out there and use your stranded prepositions: no one's stopping you!