Wheels commonly become untrue after miles and miles or riding. An untrue wheel is usually easy to spot, your wheel will like as though it is wobbling. Having a wheel that doesn't spin straight will effect your performance on the bike.
Most cycling wheels are help together with spokes over a center hub. Each spoke pulls the rim in a specific direction, putting tension on the center hub. Get these spokes to pull evenly all the way around is the art of truing a wheel.
Wheel truing can seem like a daunting task and it is if you want to match the level of expertise of a wheel builder. However, for small adjustments that aren't perfect (but good enough for the average cyclist) here is a video on how to true a wheel yourself.