Tether (USDT) is a cryptocurrency that was created in 2014. Originally called Realcoin, it was designed as a stablecoin that pegged its value to fiat currency, the US dollar, and was backed by reserves of the same amount of money.
Because market prices for many cryptocurrencies have fluctuated wildly in the past, there are many use cases for stablecoins. Most obviously, it just makes buying things easy for the consumer if commodities can be priced with some semblance of stability. It is also great for liquidity, and works as a hedge against volatility.
It's notable that although 1 USDT is equivalent to $1 US dollar, its market value can fluctuate, depending on supply and on demand.