Slippage is the difference between the quoted price of a trade and the executed price of that trade. The higher you set a slippage, the more likely the transaction may go through, and the more you may lose from the transaction.
For example, if you are doing a swap from 100 USDT to USDC, and you are quoted that you will receive 100 USDC. If you set your slippage to 10%, the least amount you could possibly receive is 100 - (100 * 10%) = 90.
This is not likely among two stablecoin pairs. But this is quite likely amongst securities that are more volatile in nature.
Setting slippage when exchanging between tokens
Setting slippage when adding liquidity into a pool