Testnet is a blockchain network used for testing the performance of blockchains. It is one of the steps that developers of blockchains have to take to test the function of a blockchain network. The test is important so as to avoid the risk of losing real money - as it is carried out without using real crypto assets. Once the testing period is completed, the network will now be launched to mainnet. Tokens/coins used for testnets have no real value, while those used on mainnet have real value. You can learn more on this site: https://blog.logrocket.com/mainnet-vs-testnet-environments-explained/