What is breakthrough bleeding?

I don't know if this makes me sound stupid but I see that all the time on Yahoo answers. Never heard that in my life. I'm not like some little 14 year old or anything I'm 24 married and have baby #2 on the way. I have never been on birth control before does it have something to do with that. Sorry just curious as I have no way to answer these questions if I don't have a clue what they are talking about.