Tether is a cryptocurrency token issued on the Bitcoin blockchain. According to the Tether team, each USDT is backed by one US Dollar. The goal is to facilitate transactions with a rate fixed to the USD.
Amongst other things, Tether looks to fix some of the legal issues which can arise when trading cryptocurrencies and it aims to protect people from market volatility.
Tether has faced constant scrutiny over the years, in particular with regards to whether or not their currency is truly backed by USD.