It means "Let's see you do it then." Don't just be "all talk" and no action. "Practice what you preach" is another saying that's similar. It's a common phrase in gambling or betting. Let's say you are at a party and someone is going on and on about how they can do 200 push-ups with one arm behind their back. So you say to them "Really, ok, well why don't you put your money where your mouth is. Twenty bucks says you can't" or another example: "Why don't you stop talking about going to Korea CJ and just put your money where your mouth is?"