What are "retainer" agreements in a freelance job?

[Btaliat]

I was just surfing the Internet and Internet when I came across a particular term , retainer. I tried to figure out what it means but I couldn't on my own. I know it to be a kind of agreement between clients and freelancers that have to do with how payment is made. But I don't really understand the concept. I believe it will be a nice idea asking my fellow BMFers to tell me what they understand about it.