A stablecoin is a promise: I will always be worth one dollar (or one euro, or one gram of gold). It is the cash layer of crypto, the thing people hold between voyages.
The promise is only as good as what backs it. Some hold real dollars in a bank, some hold crypto over-collateralised, some lean on an algorithm. When the market doubts the backing, the price drifts off the peg before anything official is announced.