’To make a long story short’ means to tell someone something with few words.
You are providing just the basic facts instead of giving a long explanation.
Antonio: Hey, how was Italy?
John: It was a little crazy but to make a long story short, I had a good time.
Leon: How are things going with your girlfriend?
John: Not good. To make a long story short, we broke up.
Leon: That’s too bad.
Sally: Have you decided what business you’re going to start?
David: Yes, I’ve been thinking a lot lately, but to make a long story short, I’m going to open a gym.