In Minetest, griefing is to destroy the building built by an other player or players without their consent.
Griefing might or might not be against the rules of a game or server. On many servers focusing on creative building, griefing is usually against the rules. But not all servers might have such a rule. Other servers/games might even make griefing part of the gameplay, like in PvP-focused games (example: Capture the Flag).