It means you should be a pool shark, but in a good way.
You're familiar with the term, right? Somebody who's so good at pool they can do what they want at the table and play exactly poorly enough to make someone think they had a good time and won legitimately.
And then they put money down and run the table, but that's beyond the scope of the analogy, not least because you don't play RPGs for money, you just play them for fun.
So the principle is a reminder: use your arbitrary amount of GM power to make sure everyone has fun.
And, yes, the principles are only for the GM. If a player goes in trying to, say, misdirect and never say the name of their move, they're going to be hoping the other players pick up on what they actually want to do in the game, and they'll be disappointed pretty often.
For further details, look back.
A lot of early hacks of Apocalypse World didn't go into as much detail about their principles and moves as Apocalypse World did, because a lot of it had been internalized as obvious. The Dungeon World explainer of the principle is pretty short, so lemme paste you the explainer from the first edition of Apocalypse World, currently available for free at apocalypse-world.com. ("Make as hard and direct a move as you like" refers to the language directing the MC, the game's GM equivalent, to make a move.)
"Make the characters' lives not boring" does not mean "always worse." Sometimes worse, sure, of course. Always? Definitely not.
The worst way there is to make a character's life more interesting is to take away the things that made the character cool to begin with. The gunlugger's guns, but also the gunlugger's collection of ancient photographs -- what makes the character match our expectations and also what makes the character rise above them. Don't take those away.
The other worst way is to deny the character success when the character's fought for it and won it. Always give the characters what they work for! No, the way to make a character's success interesting is to make it consequential. When a character accomplishes something, have all of your NPCs respond. [...] Let the characters' successes make waves outward, let them topple the already unstable situation. There are no status quos in Apocalypse World! Even life doesn’t only hurt.
"Make as hard and direct a move as you like" means just that. As hard and direct as you like. It doesn't mean "make the worst move you can think of." Apocalypse World [the setting] is already out to get the players' characters. So are the game's rules. If you, the MC, are out to get them too, they're plain [doomed]. [...]
Find what you find interesting about their characters, and play there.
So while there's nothing you can't do as a GM, there are limits to what you should do. You have unlimited license to come up with opposition, so don't hold back giving the PCs a victory. The PCs have much less ability to make up new important things about themselves, so don't break the ones they have.
This is different from not killing them - if the players do something knowing they're risking their life for it, and the dice aren't kind, they can go out, but they'll go out like heroes. It's when you decide to drop a fate worse than death on them that you should stop and think - is this me being a fan?