USD Coin (USDC) and Tether (USDT) are both stablecoins backed by US dollar denominated assets of at least equal fair value to the respective amount of each coin in circulation, held in segregated accounts with US regulated financial institutions. Such accounts are attested to (i.e. verified publicly) by an independent accounting firm. You can find those attestations here for USDC and here for USDT. These stablecoins allow users to send and receive USD-denominated value around the world using cryptocurrency infrastructure.
USDC tokens themselves are issued by the Centre Consortium, and every token can be redeemed for one dollar — effectively providing a price stable cryptocurrency. On the other hand, USDT cannot be directly redeemed for USD, and must be sold on an exchange to convert to USD.
Note that Ledn only sends and receives USDC and USDT ERC-20 tokens on the Ethereum network. Any USDC sent to Ledn by other chains will not be received by us and cannot be retrieved.
You can learn more about USDC on their official website.