ERC20 is a set of compliances for tokens on Ethereum, and EVM compatible chains.
USDT is one example of an ERC20 token. (though confusingly it can also be a TRC20, or OMNI, depending on which blockchain its on. I’m guessing that’s what they’re actually asking about)
Do you have a transaction hash? That would help us look at both what blockchain you’re interacting with, and the details of the transaction, to see what we can help you figure out.
Here’s an article with a bit more info about how to read a block explorer: