cUSDT is a type of ERC-20 token issued by the Compound protocol, called cTokens. cTokens are distributed to users who lend assets on Compound and are used to keep track of the amount of assets lent and the value of the interest accrued. cTokens accumulate interest through their exchange rate against the underlying asset. Over time, each cToken becomes increasingly valuable compared to the underlying. Each asset that is lendable on Compound has a corresponding cToken.

cUSDT is the cToken corresponding to Tether USD (USDT). The amount of cUSDT that a user holds will stay constant over time, but when you trade it back into the underlying asset (USDT), the amount of USDT will have increased based on the interest rate.