A wall hack is something that some people use when they play video games, especially first-person shooter games. It helps them see through walls and other obstacles that usually block their view.
Imagine you are playing a game where you have to shoot at bad guys who are hiding behind walls or doors. You can only see them when they come out in the open. But if you use a wall hack, it's like having a magic power that lets you see through those walls and obstacles. You can see where the bad guys are hiding, even if they think they're safe.
This is not a fair way to play the game, though. It's like cheating. So if you see someone using a wall hack, it's important to tell the game moderators or someone who can stop them from cheating. It's better to play the game fairly and have fun without using tricks to win.