A testnet is an alternative Steem blockchain, to be used for testing. A testnet is often used to prepare for an eventual hardfork. Testnet coins are ficticious and distinct from actual the MainNet tokens but it must be restarted every 90 days or so because there’s an intentionally hardcoded supply and block limit to keep the testnet token from becoming viable tokens.
There are multiple advantages of running a testnet, developers can test theirs scripts or applications without the load from the live network this in turn allows much more flexibility to try new things. Having access to a testnet also helps developers to work on new features and possibly submit new or improved pull requests to official the Steem GitHub repository.