A mainnet is a term describing an independent blockchain network, with different tokens, coins, and applications running on it. This is in contrast to a testnet, which is essentially a testing ground used to troubleshoot before projects or cryptocurrencies launch on the mainnet. In short, a testnet is the troubleshooting and testing grounds for a project, and the mainnet is where the project is launched.
For the Ethereum mainnet, it is common for soon-to-launch blockchains to first release their tokens as a token sale on the Ethereum blockchain. If successful, they are likely to then roll out their own blockchain; once it is entirely up and running, it allows users to test it out and even bridge their tokens over. This is referred to as a mainnet swap. Once the tokens bridge from Ethereum, they are typically burned or destroyed so everything is on the new blockchain.
