If people can't be made to comply with a rule or law, it's unenforceable. You can ask people not to grab greedily at the candy falling from a piñata, but your request is basically unenforceable!
If something is unenforceable, there's no reasonable way to enforce it, or make absolutely sure it's being obeyed or followed. A law that bans daydreaming would, for example, be completely unenforceable. Contracts are also unenforceable if they demand something of the signer that's not legal, like an employment contract that says you have to work 12 hours with no break.