Some kinds of fish, notably salmon, swim upstream to spawn. Sometimes, they have trouble making it all the way to salmon makeout point. Swimming upstream is swimming uphill, which is a pretty difficult thing to do for us, let alone for salmon, which have no elbows. This is made increasingly difficult by the fact that humans often build dams and other silliness which block passage to the fish and cause them great fishly anxiety.
In the spirit of generosity towards wildlife which runs rampant among humans, we have devised a device called a fish ladder. In places where people have otherwise prevented the timely upstream-swimming of fish by building a dam or other structure, an alternative route can be built so that the fish can still make it upstream. This alternative is composed of a number of 'steps', which are really just shallow terraced 'shelves' (weirs, really) which permit the fish to swim up great vertical rises just a little bit at a time. This is a huge comfort to the fish who, not having the benefit of our human intelligence, are incapable of understanding just how cool the dam really is.