Slippage happens when the price you actually get for a trade differs from the price you expected when you submitted it — usually because the market (or a liquidity pool's balance) moved between the moment you placed the order and the moment it executed.
On decentralised exchanges using automated market makers, larger trades relative to a pool's size cause more slippage, since the trade itself shifts the pool's pricing. Most DEX interfaces let you set a maximum acceptable slippage; setting it too high leaves you open to a worse-than-expected price, while setting it too low can cause transactions to fail during volatile periods.