Short answer
A sandwich attack is when a bot spots your pending trade, buys just before it and sells just after it, profiting from the price move your trade causes.
Your swap raises the price as it fills. A bot that sees it coming can buy first (pushing the price up), let your trade fill at the worse price, then sell straight after. You pay more; the bot pockets the difference.
High slippage tolerance makes you a better target, because it tells the bot how far it can push the price before your trade would fail. This is one form of what's often called MEV.
Keeping slippage as low as the token allows, avoiding very large trades in thin pools and using routes that protect transactions from public mempools all reduce the risk.
See the checks for yourself
18+ only. Self-custodial: you keep your keys. 0.5% per trade, no minimum. Memecoins can go to zero.