A fishery is the activity surrounding the capture of specific fish respective to a certain place and time. Fisheries can be commercial or recreational. Fisheries can be in freshwater, saltwater, or any body of water. The place can be as specific or broad as desired, from a very small fishing ground to all the world's oceans combined. The specific fish can be a singular species or some other logical grouping of multiple species. The method of take for capture of the fish can also be specified.
The word fishery tends to come up a lot in conversation anytime someone wants to refer to the collective effort of fishermen on fish. For example, when someone wants to speak of a historical fishery that may no longer exist. The word fishery also comes up a lot from regulators who need to describe and control the current state of fishing.
There are some precise definitions of what constitutes a fishery out there. But it is now a generalized term used by everyone from government officials to commercial fishermen, to recreational fishermen. You should be able to pick up from context-specific clues just what someone means by a fishery.
Fishing Fundamentals