Slippage is the price difference between when you submit a transaction and when the transaction is confirmed on the blockchain. Two scenarios create slippage when trading on a DEX, which leads to loss of user money.
Getting hit with slippage is common when block space is scarce, and everyone is trying to get their transaction processed. Most decentralized exchanges give you the option to adjust slippage tolerance. You can increase or decrease your slippage tolerance percentage for different situations to make sure your transaction gets picked up.
https://academy.shrimpy.io/post/what-is-slippage-how-to-avoid-slippage-on-defi-exchanges