I'm being a bit vague here because where it is now is the result of decades of arms race between the cheaters and the developers, thus extremely complicated.

It's a program that sits on your machine, and watches both the game in question, as well as other programs, to make sure there's no funny business regarding the game.

It's somewhat effective, though it comes at a cost. There is a noticeable performance cost on modern anticheat programs, meaning you're losing performance for sometimes no tangible benefit.

Kernal level anti cheat programs are especially invasive, as they have deeper access to your operating system than would normally be allowed by any other program. This can be fine when used correctly, but if abused could be a massive privacy concern, or even a danger to your system. If you remember a little while back where every business windows computer went down for a day due to the crowdstrike antivirus, that's a potential result of kernal level software going bad.

So somewhat necessary evil, and they do kind of work. I personally don't love solutions that eat performance like this, and privacy is a concern of mine, but also I kinda get it.

Anticheat is software that tries to identify cheating while you're running a game. Kernel level anti cheat is malware.

Anti-cheat is software made to prevent cheating. In order for it to be effective, you basically have to surrender control of everything in your computer to the Anti-cheat software and just trust that they won't fuck you using that.

Last year for example, an Anti-cheat software got hacked and it installed cheats into player's computers.

From my understanding, it's either client or server side software for online multiplayer games to try and prevent people from using 3rd party tools or hacks to gain an advantage from ruining the experience of other players. I can't give a more technical answer, though, as to how it works. Someone else would have to provide that.

Essentially, most cheats for games work because the program can access the RAM addresses that the game uses. Anticheat works by scanning the computer for these running programs/services that are known to be cheats.

Historically this has been done in userspace, ie. no elevated permissions. Nowadays, Kernel level AC let's the AC check for deeper cheating methods, like devices that are operating on a driver level.

Currently, the most difficult to detect method is cheating using a 2nd PC that connects via a cable to a special PCIe device in the gaming PC. It essentially analyzes everything going to RAM and plucks out game related info. It's currently a back and forth trying to hide that PCIe device from the anti-cheat.