It means validate your idea quickly against real customers and do something else if they don't love it. Once you determine to do this, you find that many ideas you thought were great are total shite, so you fail fast because you don't do much development without validation. You are able to iterate through multiple concepts quickly until you arrive at one people love, and then you throw yourself behind it and get it built and get people paying.
The common mistake is to pick an idea out of your head, spend six months building it, and then find out nobody is willing to pay for that thing over the next six months after that.
Thats what fail fast means. Its the key to success.
I'm still not sure what it means -- I hope it's fairly synonymous with "fail cheaply", or to me at least it's useless advice.