The mainnet (short for main network) is the original and functional blockchain where actual transactions take place in the distributed ledger and the native cryptocurrencies possess real economic value. In other words, the mainnet refers to the actual open-sourced blockchain itself that is publicly verifiable.
The mainnet carries out the functionality of executing real transactions within the network which is stored on the blockchain and is referred to as the "end product" that is open for the public to use. Each transaction executed on the blockchain requires participants to pay a transaction fee (payable in the native coin) so as to incentivize miners to validate the transactions and prevent network spamming. For their hard work, miners will be rewarded with native coins from the protocol and also the transaction fees paid by the participants.
You can access Waves mainnet via a full node. it's integrated with the GUI in the Waves client, allowing anyone to use it without any technical knowledge or blockchain downloads.