Slippage is the difference between the price that a swap is estimated at and the actual price when the trade executes. This can be caused by a swap being placed and the price changes in the time between the order being created and executed.
It will also happen when an order is large enough that it is split between many trades. You can adjust this setting in the “Rate” section of the quote by pressing on the pencil icon and choosing between 0.5%, 1%, 3%, and your own custom percent. The default setting is 0.5%.