Is USDT backed by anything?

Good afternoon, I am relatively new to the world of cryptocurrencies and I am trying to wrap my head around the concept of stablecoins. One particular stablecoin that has caught my attention is USDT. I understand that it is pegged to the value of the US dollar, but what exactly backs it? Is it fully collateralized? If so, what assets are being used as collateral? And how do we ensure the collateral is sufficient to back the issued USDT? Thank you for your time and assistance.