GlossaryImpermanent loss
Impermanent loss is the difference in value between providing two tokens to a liquidity pool and simply holding them, which appears when the tokens prices diverge.
It is unrealized while you stay in the pool and reverses if the prices return to their entry ratio. It only costs you if it exceeds the trading fees you earn, so stablecoin and correlated pairs see the least and volatile pairs see the most.
Try the impermanent loss calculator to see it on your own numbers.