These are websites/services where you can exchange various crypto currencies ... buy buying or selling them. There are two major types : centralized and decentralized exchanges.
Centralized exchanges are managed by some party/company and they serve as the intermediary, some of them also offer the option to buy crypto currency for regular FIAT money. Decentralized Exchanges work on a peer-to-peer basis, connecting buyer with the seller directly.
Every exchange supports certain range of crypto currencies, some allow to trade many coins (like Binance), while some only focus on the major ones (like Coinbase).