Tether is a cryptocurrency that should always be worth 1USD (with occasional tiny variations due to selling and buying pressure) Its built on the Omni Layer blockain and is run by a company of the same name, Tether.
The exchange Bitfinex is a sister company of Tether so both are deeply connected. Tether is used on all major cryptocurrency exchanges with few exceptions (Kraken, GDAX, Bitstamp).
Tether should be always worth 1USD becaused Tether/Bitfinex should be storing 1USD for every 1USDT. Usage wise users are buying USDT on exchanges with mostly Bitcoin and Ethereum (other coins are rarely used) when the price is falling and market crashing.
The aim of that strategy is to sell BTC/ETH at high price and then buy back when the price is lowered. This works because USDT is always the same price even during market crashes