A fishing net or fish net is a net used for fishing. Fishing nets work by serving as an improvised fish trap, and some are indeed rigged as traps (e.g. fyke nets). They are usually wide open when deployed (e.g. by casting or trawling), and then close off when retrieved to engulf and trap fish and other aquatic animals that are larger than the holes/gaps of the net, as well as many unwanted bycatches due to the underwater area a net can cover.
Fishing nets are usually meshes formed by knotting a relatively thin thread, and early nets were woven from grasses, vines, flaxes and other fiber crop material, while later woven cotton was used. Modern nets are usually made of artificial polyamides like nylon, although nets of organic polyamides such as wool or silk thread were common until recently and are still used.