The mainnet of Bitcoin is much like a computer network and is the main network where Bitcoins are sent and received. It is the decentralized network that allows transactions to be broadcast from one user to another without going through a third-party intermediary like a bank.
The records of all Bitcoin transactions are stored in a public ledger called the blockchain. The mainnet acts as a peer-to-peer network like BitTorrent, where users directly connect with each other in order to share data. When a Bitcoin transaction is sent, it is processed by the global network of computers connected to the mainnet using the protocol.
Every time a new transaction is broadcast, the computers connected to the mainnet working to process it, known as miners, have to solve a cryptographic puzzle in order to verify its legitimacy. Once a block has been successfully processed, it is added to the blockchain permanently and can never be changed or removed.
The mainnet is incredibly important for Bitcoin because it is the only way for people to transact with each other and ensure that their transactions remain secure and trustworthy. Without it, Bitcoin would be useless for both users and miners, because there would be no way to guarantee the accuracy or consistency of transactions that were sent and received.
Additionally, by running on a decentralized network, Bitcoin can avoid relying upon any individual or entity to regulate or validate its transactions. This makes it much harder for governments or corporations to manipulate or control Bitcoin in any way.