That's ok. We all have questions, and newcomers have many of them, for sure. CEX means centralized exchange. For example, Binance, Coinbase, OKX, etc. They act as intermediaries between you, the customer, and the blockchain world, and usually hold your funds in custody. DEX means decentralized exchange. DEXes don't have an intermediary between you and the blockchain world, they don't take your funds in their custody (funds remain in your wallet or in smart contracts you enter by your own choice), but you need to keep your private key(s) safe and be careful what operations you approve. You are responsible for the security of your own wallet.
That's ok. We all have questions, and newcomers have many of them, for sure. CEX means centralized exchange. For example, Binance, Coinbase, OKX, etc. They act as intermediaries between you, the customer, and the blockchain world, and usually hold your funds in custody. DEX means decentralized exchange. DEXes don't have an intermediary between you and the blockchain world, they don't take your funds in their custody (funds remain in your wallet or in smart contracts you enter by your own choice), but you need to keep your private key(s) safe and be careful what operations you approve. You are responsible for the security of your own wallet.