Short answer
Price impact is how much your own trade moves the price in the pool. The bigger your order relative to the liquidity, the bigger the impact.
Decentralized exchange pools price tokens by the ratio of what's inside them. When you buy, you take tokens out and put SOL in, which raises the price for the next unit, including the later part of your own order.
Small trades in deep pools barely move anything. Large trades in shallow pools can move the price by double-digit percentages, which you pay for on the way in and again on the way out.
Price impact is separate from slippage, which also includes other people's trades. Splitting a large order, or choosing tokens with more liquidity, reduces it.
See the checks for yourself
18+ only. Self-custodial: you keep your keys. 0.5% per trade, no minimum. Memecoins can go to zero.