A fillet is a specially cut piece of meat or fish. If you order a fillet of salmon at a restaurant, it won't have any bones in it.
You can eat a fish fillet or a fillet of beef — either one will be boneless. If you cut a fillet yourself, you fillet the meat. The oldest meaning of the noun fillet, one that's rare today, is "a ribbon worn around the head." It comes from the French filet, the diminutive form of fil, or "thread." The meat meaning came from the technique of tying it with a string before it was cut.