A fluke is an unexpected stroke of good luck. It was a fluke to find that fifty dollar bill on the ground, and it made you smile for the rest of the day.
The word fluke was first used in 1857 in reference to a lucky shot at billiards. If something good happens to you by chance when you're not expecting it, that's a fluke. The word fluke can also be used in a negative or insulting way. For example, if your favorite sports team loses an important game, you could complain that it was just a fluke that the other team won — meaning they didn't deserve to win and somehow got lucky.