Arbitrum is a network designed to enhance the Ethereum blockchain in terms of speed scalability while adding additional privacy features to boot.
Arbitrum is a layer-2 solution. Layer 2 networks (like Optimism) are not separate blockchains. Instead, they combine large quantities of transactions off-chain (off of the Ethereum blockchain) before submitting them bundled together on-chain, thus relying upon the same security and consensus mechanism as Ethereum.
You can read more about Arbitrum in the official blog.