Besides the main Bitcoin network, called the mainnet, there is also a testnet where new projects and protocol features are tested. The testnet is an alternative blockchain used for testing and research. The testnet is a security measure to ensure that the main Bitcoin network is never jeopardized in the process of experimentation. In this network, we use Testnet coins.
Testnet coins are separate and distinct from the actual mainnet Bitcoins. They aren't supposed to have any value. This allows application developers or testers to experiment, without having to use real Bitcoins or putting the main blockchain at risk of faulty software.