OP Mainnet lets you send transactions, similar to Ethereum, but with two important advantages, and one feature that stays exactly the same.
Faster transactions. OP Mainnet produces blocks every 2 seconds (as opposed to every 12 seconds on Ethereum).
Much lower transaction fees. Typically between a 1% and 10% of the cost on the Ethereum mainnet (also known as layer 1 or L1). For the current gas price and a few sample transactions' costs see here.
Decentralization: All transactions are posted to L1 Ethereum, inheriting the strong security guarantees of Ethereum.