Birth control while breast feeding

I'm due Oct 6, and I asked my doctor if I breast feed if I will still be able to take the pill. She told me I could take what they call a "mini pill". Which means it doesn't have any estrogen in it. When I asked her what that means, she said I would have to take it at the EXACT same time everyday, or it wouldn't be as affective. What does that mean that it doesn't have any estrogen in it? Why does that make a difference? Because I can not guarantee I'll take it at the same time everyday. But I don't want another baby for quite a long time.