A cryptocurrency exchange is a platform where you can buy and sell cryptocurrencies. You can exchange some coins for others or buy them in exchange for fiat, that is, real money. For example, you can convert bitcoins to litecoins, pay dollars for them, or withdraw cryptocurrencies into cash. I recommend using the Bybit broker as it provides good commissions. You can find more information about bybit exchange fees here https://tradersunion.com/brokers/crypto/view/bybit/bybit-fees/