In a nutshell, wheel alignment means adjusting the angles of the wheels so that they make a perpendicular angle to the ground and stays parallel to each other. The purpose is getting maximum tire life and a vehicle that has straight and true tracks when driving along a straight and level road.
If your vehicle tracks like an arrow, the wheels likely are properly aligned. Even so, it is good to check alignment when you have your tires rotated. Don’t forget to align the wheels when you buy new tires so they don't immediately start to wear unevenly.