The term Mainnet refers to the phase wherein the blockchain protocol is fully developed and deployed. What this entails is that all transactions for a cryptocurency is broadcasted, verified, and duly recorded on a blockchain.
Prior to the launch of a blockchain project’s mainnet, an Initial Coin Offering (ICO) or an Initial Exchange Offering (IEO) or any other method to raise funds and develop the community should be implemented. The funds will then be used to develop the prototypes of the blockchain. After the bugs have been fixed and, the mainnet version of the blockchain network will be launched, provided it could already function well.