Optimism lets you use Ethereum for about ~$0.5 per transaction instead of the ~$15 you'd pay on Ethereum Mainnet.
Optimism is like a different bank account. You'll need to move your money over, but when you do, every transaction is much, much cheaper.
Moving money from Mainnet to Optimism
To move money from your Mainnet account to your Optimism account, use the Optimism Bridge.