Polygon is a Layer 2 scaling solution for Ethereum that we introduced on 11/15/21 for minting and managing domains on our website. All domains minted after this date are done so using the Polygon - a separate blockchain that uses the same codebase and wallet addresses as Ethereum. The reason for the implementation of Polygon was due to the increasing traffic on the Ethereum mainchain making the usage of Web3 Domains expensive due to high gas fees (or transaction fees). Since introducing Polygon as the primary way to mint and manage domains, we are able to continue covering all gas fees for users on our website.
You can read more about why you should use Polygon here.