Sweep means to buy all the listed NFTs in a collection at the current floor price. The floor price is the lowest price an NFT in the collection is listed at. When a buyer or project buys all the floor price NFTs, they are sweeping the floor.
Since you are buying multiple NFTs in one transaction, sweeping the floor allows you to save on network fees.
Here’s how you can sweep a NFT collection using Uniswap.