A mainnet (short for "main network") is a fully operational, independent blockchain where real cryptocurrency transactions are broadcasted, verified, and permanently recorded on the distributed ledger. It is the live production version of a blockchain project, as opposed to a testnet which is used for development and experimentation. When a project "launches its mainnet," it means the network is ready for real-world use.
Mainnet
Mainnet Explainer Video
What is a Mainnet? | Crypto Terms Explained