The mainnet refers to an individual blockchain network that runs within its own technology and protocol. The mainnet blockchain network often comprises its own cryptocurrencies and tokens that are heavily used.
This is completely different in contrast to a testnet where any given project or token runs on top of a preexisting blockchain such as Ethereum. Testnet is often used by programmers to diagnose and troubleshoot new blockchain features.
Mainnets are completely developed blockchain projects, as compared to testnets that often run at the most cutting-edge technology. The mainnet phase of a blockchain is always rolled out, after rigorous testing in the testnet.
Once a project moves to its mainnet phase, it can put out its native tokens instead of using the ERC-20 tokenization standard from Ethereum.