Which role do fish ladders serve

Its so that migrating fish can get by a dam that is in the way ... Usually its a dam sometimes its a different sort of barrier

A fish ladder is also known as fishway. This is a structure which is build artificially to allow the diadromous fishes natural migration from one place to another.

This structure is usually build around dams, waterfalls or locks. The velocity of water is kept enough so that the fishes by swimming and leaping.

Most of the fishes spend their life in combination of water. So, fish ladder is built for those fishes so that they can move and spend their cycle of life in both types of water.