What is BTC USDT?

Excuse me, I'm new to the world of cryptocurrencies. Could you please explain what BTC USDT is? I've heard a lot about Bitcoin (BTC) but I'm not sure what USDT is and how it relates to BTC. Are they both cryptocurrencies? And how do they differ from each other? Is it safe to invest in them? Could you also tell me about their potential returns and risks? Thank you so much for your help!