It means you have to take action and prove you can do something that you said you could do. "Jacob said he could learn English in only a year." "Now he has to put his money where his mouth is."

This originates from the idea that if someone really believes something than they would be willing to bet their own money on it, either in the context of investment or gambling. Often this phrase is used to challenge someone to make a bet on something about which two people are disagreeing. It also has a more general meaning of asking someone to prove their convictions by attaching money, time or reputation to the claim. Two good examples might be: * I think Liverpool will win the league - Why don't you put your money where your mouth is? I'll bet you £50 they don't! * It's easy to run a marathon - Why don't you put your money where your mouth is and enter the race?

It could also have to do with living up to your stated beliefs and convictions. e.g: "You have always claimed to be against consumerism. Now that everyone is buying fancy electronic gadgets, put your money where your mouth is."