Slippage tolerance is a setting for the limit of price slippage you are willing to accept. By setting slippage tolerance, you basically set a limit on how many tokens you will accept, whether higher or lower than the rate shown in the interface. Slippage tolerance is set as a percentage of the total swap value. For instance, if you set slippage tolerance at 3%, it means that the amount of tokens you will receive will not differ greater than by 3%.
For your convenience the 'slippage tolerance' indicator can be pinned to the swap page by clicking on the star icon next to it in the settings.