That’s the sort of general question where I couldn’t give you a better answer than you could find by Googling if I tried. But for my own amusement, a one sentence answer is that it’s a sort of mega command that lets you do various types of rewriting history like removing, reordering, redoing, rewording.

I recommend using a good GUI for it. That’s usually a lot easier than vim or another text editor if you’re not ready to go down that rabbit hole.