A more thorough explanation:

Definition: A dam made of stakes interlaced by twigs of willows that are placed across a river to more easily accommodate the netting of fish. Also termed weir.

Example: The fishermen built a wear across the river to catch more fish.

This definition explains that a wear is a type of dam made of stakes and twigs that is used to catch fish. The example sentence shows how fishermen use a wear to catch more fish by creating a barrier in the river that makes it easier to catch them.