|A DEX is a Decentralized EXchange|
It's a place where you can Exchange one token or coin for another.
Uniswap is a popular one.
You must convert your currency like US Dollar to a stable coin first exchanging for a different coin.
The fees for convert an Ethereum token can be high right now.
You'd use a DEX when the token you want isn't traded on the exchange you're using or if you don't want to go through KYC (Know Your Customer) or AML (Anti-MoneyLaundering)