Ladders are just a way of matching people. It records all you wins and adds them up. As you win more matches, you rise up the ladder. But say, if you play someone on a low rung and win, you'll rise a few places, but if you play someone on a higher rung (as the nearer the top you are, the generally better you are) and win, you'll rise up quite a lot of places.
Overall its just a fun way of regularly playing people and its down to you whether you play someone not as good, about the same, or better than you!