USDT is a token running on the Ethereum blockchain that is pegged to the value of a U.S. dollar (this means that 1 USDT shall always be worth 1 US dollar). As a result, USDT is a so-called stablecoin (it does almost not change in price). USDT is also known as Tether or Tether USD. Tether’s issuer claims that USDT is backed by bank reserves and loans which match or exceed the value of USDT in circulation.
USDT is also running on a number of other blockchains than Ethereum. Be aware that you should always send USDT within the respective blockchain you are running at. For example, you cannot send USDT running on Ethereum (ERC-20) to USDT on the Binance Smart Chain (BEP-20).