Something far-fetched 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 far-fetched: A far-fetched idea or plan is implausible because there's very little chance it will happen. Flying to Jupiter is far-fetched. Marrying a movie star is a far-fetched idea, and so is becoming a movie star. Far-fetched things aren't necessarily impossible, but they're very unlikely. Children are especially good at coming up with far-fetched notions because of their powerful imaginations. Note that this word is sometimes unhyphenated as farfetched.