USDT is a crypto-token issued by a company that advertises with a 1:1 peg to the Dollar, but has no means to make good on this (they lack usable banking partners just like Bitfinex) and they explicitly reject any guarantees in their terms of service.

USDT is reasonably stable around $1, so it can be used as a temporary way to get out of crypto or to transfer funds between exchanges, but don't rely on it keeping its value.