An NFT marketplace is a market to buy, sell, trade and manage NFTs using a Web 3.0/Crypto wallet.
Good marketplaces also show you stats (rankings and activity) on NFT collections.
Exmples of NFT marketplaces:
OpenSea.io
niftygateway.com
Rarible.com
AtomicHub.com
These are just a few NFT marketplaces, there are a ton!