The testnet is a separate blockchain from the Stacks mainnet analogous to a staging environnement. It's a network used by developers to test their apps, smart contracts, or changes to the protocol in a production-like environment.
It produces blocks at roughly the same rate as mainnet; about 1 block every 10 minutes on average. The Stacks testnet is rarely reset.
Testnet faucets provide you with free Stacks Token (STX) to test with. These are not the same as STX on mainnet and have no value. There are a couple of different options for getting testnet STX.