Slippage is the difference between the price you expected when placing a trade and the price it actually executed at. It happens because prices can move in the brief moment between when you submit an order and when it fills, or because there are not enough orders at your desired price.
Slippage tends to be larger in low-liquidity markets or during periods of high volatility. If you place a sizeable order in a thin market, filling it may require reaching worse prices, moving the average price you pay or receive.
Many exchanges and apps let you set a slippage tolerance, which limits how much difference you are willing to accept. Understanding slippage helps explain why a trade sometimes completes at a slightly different price than the one first shown.