Look it up in the dictiannary. It’s when you stop playing and say your opponent won.
What is resignation
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5.1.2 The game is lost by the player who declares he/she resigns (this immediately ends the game), unless the position is such that the opponent cannot checkmate the player’s king by any possible series of legal moves. In this case the result of the game is a draw.
https://handbook.fide.com/chapter/E012023
resignation