To paraphrase Pekelo, I hate to be that guy, but here goes.
Tether (USDT) is a stablecoin. It's not the only one, there are others, USD Coin (issued by Coinbase), TrueUSD (TrustToken). Each token is
backed by an equivalent $ amount. These are centralized coins. Think of the issuers as "mini central banks" printing their own money. If there is trust in the company behind the stablecoin or the overall counterparty risk, then the price is stable at $1 per token. If not, then the price will go down to less than $1. As far as I'm aware, all the stablecoins are stable at this time but they were not stable during the crazy times we had mid-march.
Tethers are what I would equate to money market funds like when you sell stocks at a broker, but in this case, you sell cryptocurrencies at a crypto exchange. Some stablecoins give you interest.
I wish that I didn't distrust USDT or all stablecoins during the bull market, I could have cashed out 6 figure amounts in profits and held the value in stablecoins instead of riding the bear market down. Will be ready to utilize them on the next cryptos bull market.
https://changelly.com/blog/best-stablecoins-comparison/