A Testnet describes a blockchain that isn’t fully operational and deployed to a production environment.
Testnets, as the name implies, are used strictly for prototyping or testing a blockchain protocol or project.
Testnets are great for troubleshooting bugs, testing all features of the blockchain, and confirming that the system is secure before launching to mainnet.
Since testnets are completely separate from the mainnet, they can run in parallel to the mainnet.
Testnets aren’t monetized, so no funds are at risk and no fees are paid.