Price Impact is the effect a Buy or Sell Order has on the automated market maker (AMM). The bigger the trade, the bigger the effect on the AMM.
Example: A large order to buy BTC will drive the price of BTC up and conversely a large order to sell would result in the price going down (within the specific DEX you are trading with). It's important to keep an eye on the 24hr volume of the particular cryptoasset you intend to buy/sell/trade so you can get an idea on how much your transaction will impact the market.