BTC - means bitcoin
USDT- means "Tether" which is a cryptocurrency stable coin (designed to be more stable than other regular cryptos)
So, BTC/USDT is a cross between Bitcoin & Tether.
On the other hand, BTC/USD is a cross between Bitcoin and the US Dollar.
For me i trade BTC/USDT because the spreads are lower than BTC/USD. But, if you are a beginner you are better off trading BTC/USD which is less volatile than BTC/USDT