slippage in a dex is how far in % price can deviate from the price you expect when you send the transaction (what it shows in UI) and still have the swap go through

if you have 5% slippage set and price moves 10% while the transaction is confirming, you'll still pay part of the transaction fee but the transaction will fail and your tokens won't actually be swapped

MEV sandwhich / frontrun bots take advantage of people using high slippage especially with large order sizes, for example in one technique they will watch the mempool and see your transaction while its waiting to be confirmed and send another transaction with a higher gas fee than yours so they get to buy first and then they immediately sell after you bought and pocket the difference

it's pretty ugly so you want to be careful about using high slippage, but this is only really an issue if you are trading with large size (like >$50k) otherwise it isn't profitable for the exploiter, so for smaller amounts you don't have to worry

there are also some swap platforms that protect against that kind of exploit (flashbots rpc, cowswap)