Optimism is Ethereum's next biggest Layer 2 to date (only beaten by Arbitrum). A Layer 2 blockchain is an extension of Ethereum that benefits from the security assurances provided by Ethereum while operating independently. It can do all of this because it's an Optimistic Rollup.
Please check your link for Velodrome twitter. The first instance of it is linked to a fake account https://twitter.com/velodromefi_
It has an underscore as suffix