Is Optimism built on Ethereum?

Could you clarify for me if Optimism is indeed constructed upon the Ethereum platform? I've heard rumors about this, but I'm not entirely sure. It seems to be a topic of much buzz in the crypto community, and I'm eager to understand the intricacies behind it. Is it a layer-two solution that leverages Ethereum's scalability and security? Or is it something entirely different? Your expertise in this field would be invaluable in shedding light on this matter for me.