Slippage
The gap between the price you see and the price your trade actually executes at.
You click 'buy ETH for 3000 USDC', but by the time the tx hits a block the price moves or you eat into the DEX pool, so you buy at 3015. That's 0.5% slippage. For small buys on liquid markets (BTC, ETH) it's 0.1-0.3%. For new memes it can reach 5-20%. Always set a slippage limit, because without it MEV bots can squeeze you (sandwich attack).
In plain words
You click 'buy', but by the time the tx lands in a block, price slips a bit. You get a slightly worse rate than you saw. On BTC/ETH it's pennies, on small memes it can be over 10%. Always set a max slippage limit, otherwise MEV bots will squeeze you.
Example
Buying 50 ETH from a small ETH/USDC pool with 0.5% slippage, you'll actually pay 1-2% more.