What is Rinkeby?

Rinkeby is an Ethereum testnet (or test network). Testnets are typically used by developers to run "tests" for their application or software. Currency on testnet are valueless. Rinkeby testnet, unlike Ethereum mainnet, is a proof-of-authority network, as opposed to a proof-of work network like Ethereum mainnet. Stats for the Rinkeby testnet can be viewed here.