Something farfetched is imaginative but very unlikely. It's a lot easier to think of than to do.
Our brains can think of all sorts of ideas. Some are realistic, like going to the store or getting married. Others are farfetched: a farfetched idea or plan is implausible because there's very little chance it will happen. Flying to Jupiter is farfetched. Marrying a movie star is a farfetched ideas, and so is becoming a movie star. Farfetched things aren't necessarily impossible, but they're very unlikely. Children are especially good at coming up with farfetched notions because of their powerful imaginations.