When you fight, you dispute or argue. Everyone disagrees from time to time, but it's sad when close friends fight.
The verb fight means to engage in a struggle that involves conflict — and as a noun, fight is the conflict itself. A fight can take a physical form, like a boxing match or a playground skirmish, or it can happen with words, like a fight over politics. The ancient root of fight comes from the Proto-Indo-European prefix pek, meaning "to pluck out." Picture a hair-pulling fight, and this makes complete sense.