Most trading on crypto exchanges is priced against stablecoins such as USDT (Tether) and USDC. Issuers hold reserves to back them and redeem them at face value.
Why it matters
Stablecoin supply is a rough gauge of money ready to buy crypto. Their reserves have made issuers large holders of US Treasury bills. A stablecoin losing its peg, as several have, can trigger wider crypto selling.
General information, not advice.