TRC-20
The type of token used on the Tron network. 'USDT TRC-20' means USDT on Tron.
In plain words
Tokens on Tron follow a standard called TRC-20, the same way tokens on Ethereum follow ERC-20.
Why it matters if you've been scammed
If you were asked to pay "USDT TRC-20", your money is on Tron. See how to trace USDT on Tron.