In hunting and fishing, open season is a time during which it is legal to capture or kill a certain kind of animal. Typically hunting/fishing seasons are annual or semi-annual; even during the season there is typically a limit on the amount of creatures you can hunt, trap, or fish.
Open season is also an idiom used to exaggerate criticism or negative feelings toward a person. For example:
"It's open season on deer, so we went hunting for the weekend," or
"Why is everyone attacking me at once? I feel like it's open season," or
"George W. Bush has declared open season on terrorists."