Mainnet is the term used in blockchain technology to refer to the production or live network of a cryptocurrency or blockchain platform.
It is the operational network where real transactions occur, as opposed to a testnet or a development environment. The mainnet is the fully functional and decentralized version of a blockchain network, where users can send, receive, and validate transactions, and where tokens or coins have real economic value.
MetaMask is a cryptocurrency wallet and browser extension that allows users to manage their digital assets and interact with decentralized applications (DApps) on the Ethereum blockchain.
Mempool (short for "memory pool") is a component found in the blockchain serving as a temporary storage area for pending transactions awaiting validation and inclusion in the next block of the blockchain.