Actually, a ladder (in gaming circles) is commonly thought of as a contest.. a competition (like a "clan ladder") in which you play against people of higher rankings (based on wins) and "climb" your way to the top.
If you beat somebody lower on the ladder than you, you get fewer points (or none) and you don't go up much (or at all). To rise to a higher rank, you have to beat somebody of higher (or sometimes equal) rank who is worth more "points."
It depends on the rules of the competition, but that's basically how it works. Eventually a winner is declared (the person on top after all the rounds have passed).
It's something that is setup outside the game, by humans, but some games have "ladders" built in.
The reason why they call some of the addon JK2 SP levels "ladders" is because it is a scenario in which you face a series of opponents of increasing difficulty that you fight, as if you were a participant in a "ladder" competition. A lot of games have those, but I usually don't see them referred to as "ladders." Usually "survival mode" or "training" or "challenge" or "time attack" something like that, but that's the basic idea.