Earlier quoted context omitted.
The promise of a stablecoin, in this case Tether, is that you can always change 1 USDT into $1. When people buy Tether, they pay then $1 and believe the promise that they will get $1 when they want to.
Right but how... where does physical money come into play? How can I or anyone get 1 physical dollar from selling USDT? All I've ever done is exchange some coin => USDT and vice versa. So if no one is actually asking for a physical dollar, why does it need to be backed by a physical dollar?
a) 1 USDT for a USD, and
b) 1 USD for a USDT.
Any USDT/USD deviation from 1 thus gives rise to arbitrage opportunities. Tether can guarantee
a) because they can issue an unlimited amount of USDT, so when you give them 1 USD, they can definitely give you 1 USDT, and b) because
b1) they do not issue USDT without someone giving them the equivalent amount of USD, and
b2) they keep those USD safe in cash, deposits, or equivalent, without price, credit, and FX risk.
a) is undisputed. b) is the tricky part, both
b1) - they could just issue USDT without having received USD. Who's checking?
b2) - they could just take the USD they've received and buy a yacht, or put it in shares (price risk!) or commercial paper (credit risk!) or Chinese Yuan (FX risk!) or anything else. Who's checking?