Polygon is a totally distinct and separate blockchain that is compatible with Ethereum. It uses a different consensus mechanism than Ethereum and has its own security measures. Polygon is also known for lower transaction fees and faster transaction processing.
Polygon is Ethereum compatible, meaning that your wallet address on Ethereum is the same as on Polygon. Assets can be moved back and forth between Polygon and Ethereum using a “bridge”.
You can read more about Polygon in the official blog here.