The trading pair is a combination of two cryptocurrencies - you can trade them against one currency. For example, BTC/USDT means that you are converting BTC to USDT or USDT to BTC.
Common pairs with high liquidity:
✔️ BTC/USDT
✔️ ETH/BUSD
✔️ SOL/USDT
Sometimes, for specialized coins, you must first purchase the base currency (like USDT) before entering. It is important to understand trading pairs for good planning of your entry/exit.