The action of causing someone grief in a video game.
Common in Call of Duty, Minecraft and a few other video games.
In minecraft it is using hacks to destroy something that someone has made, or just use hacks to kill them.
In COD it's using stuff like fly, forcefield, aimbot and ESPs.
Do not attempt griefing in real life, you may get hurt by the person you are griefing.

A griefer is a player in a multiplayer video game who deliberately irritates and harasses other players within the game (trolling), using aspects of the game in unintended ways. To qualify as griefing, a player must be using aspects of the game in unintended ways to annoy other players.

In racing games it could be driving the wrong way.
In shooters it could be killing your own teammates.

Simply playing the game in a way it isn't meant to be played, just to annoy other players.

Griefing is luckily a bannable offence in most online multiplayer games.