Tether (USDT) is a digital dollar – a stablecoin pegged to US dollar. Stablecoins are a type of cryptocurrency with a value fixed to other assets (usually assets outside of the cryptocurrency space, e.g. fiat currencies, precious metals, etc.).
A Stablecoin is a cryptocurrency with a fixed-price by means of being pegged to other assets like fiat currencies, gold, precious metals and other cryptocurrencies. They have plenty of use cases including ,but not limited to, being used as a non volatile store of value to investors, earning interest on, transferring between exchanges, and lending.