CEX stands for Centralised Exchange. CEXes are online trading platforms where buyers and sellers are matched together via an order book. They function the same way as online stock brokerages. You must have a funded account to conduct any trades, and typically you are required to prove your identity before having full access to all functions. Important to note that on a CEX, you do not have full custody of your crypto assets. The exchange may withhold your funds, given that they have a valid reason for it. Remember: Not your keys, not your crypto.