Short answer
Slippage is the difference between the price you expect and the price you actually get. A slippage setting is the most you're willing to accept before the trade is cancelled.
Between pressing buy and your transaction landing, the price can move, because other trades hit the pool first or because your own order is large relative to it. The gap is slippage.
A slippage tolerance of 2% means: fill my order only if the final price is no more than 2% worse than quoted. Too low, and fast-moving trades keep failing. Too high, and you can be filled at a terrible price or targeted by sandwich bots.
For thin, fast memecoins, people often raise tolerance. Do it knowingly, and never set it higher than the loss you'd accept on entry.
See the checks for yourself
18+ only. Self-custodial: you keep your keys. 0.5% per trade, no minimum. Memecoins can go to zero.