The 20 amp Bussmann fuse?

This question has probably been asked a million times, but ive searched everywhere and cannot find out what the 20 amp Bussmann fuse on the positive terminal of the battery controls. I do know when it pops the engine shuts down. I've been told electrical system but it cranks over just won't start, I've been told fuel pump. I figured if anyone knew it would be here and I couldn't find anything in posts or archives. Mine keeps popping after changing it and driving. I'm in the process of eliminating it and just using an inline fuse