What is a mainnet?

The easiest way to describe a mainnet is by comparing it to a testnet. A testnet is a blockchain network that is not yet up and running. It is used by programmers and developers to test out new features or search for problems in their application or blockchain network. 

In comparison, the mainnet is the term given to the main network that is fully operational, with miners or verifiers going about their day-to-day business adding blocks to the chain and facilitating payments. For example, when people refer to the Bitcoin network, they are usually referring to the mainnet.