Impermanent loss
The shortfall of providing liquidity compared with simply holding the same tokens.
In plain English
It grows with how far the price ratio moves in either direction, and becomes permanent when you withdraw. Fees earned may or may not make up for it.
Example
In a standard 50/50 pool, if one token doubles against the other, the position is worth about 5.7% less than holding would have been, before fees.