USDT is a pegged cryptocurrency.
Its value is pegged directly to that of the US Dollar. Tether, the company responsible for issuing USDT in circulation, keeps the US Dollar equivalent of all USDT value in reserves at all times. Otherwise, they could never guarantee the value of one USDT being equal to US$1.
Tether has run into some banking issues a while ago. The company uses a Taiwanese bank to receive and send out USD withdrawals. Unfortunately, that bank raised some issues, resulting in the company being unable to convert USD to USDT and vice versa. This forced users to rely on third-party exchanges to complete this conversion process. At one point there was a 7% fee to do so, resulting in one USDT being worth only US$0.93