In contrast to mainnet networks, the term testnet describes when a blockchain protocol or network is not yet up and running on its full capacity. A testnet is used by programmers and developers to test and troubleshoot all the aspects and features of a blockchain network before they are sure the system is secure and ready for the mainnet launch.
After the ICO crowdfunding is complete and the blockchain is fully deployed, the team may release their mainnet, which will have their own native coin (on a proprietary blockchain) rather than the previously issued ERC-20 token. At this point, a process known as mainnet swap takes place, where the ERC-20 tokens are swapped for the coins of the new blockchain. After the mainnet swap is completed, remaining tokens are usually destroyed so that only the new coins can be used.
Despite the popularity of Ethereum and the ERC-20 standard, there are many other blockchain platforms that support the issuance of digital tokens (e.g., Stellar, NEM, NEO, TRON, and Waves).