What is Slippage?
Slippage is the gap between the price a trade is quoted at and the price it actually executes at, caused by pool depth, price movement between quote and confirmation, or transaction ordering.
In practice
Most interfaces let users cap acceptable slippage, which protects against thin liquidity and against sandwich attacks where a bot trades either side of a pending transaction. Wide tolerances on illiquid pairs are one of the most common ways users lose value without noticing.