A faucet is a website or app that provides a small amount of tokens at the request of a user. Users provide the faucet with an address where they wish to receive tokens. This is typically to facilitate participation in testnet applications.
The first bitcoin faucet, called The Bitcoin Faucet, was developed by Gavin Andresen in 2010. It originally gave out 5 bitcoins per person. Since then, the faucet concept has been applied to testnet tokens and other blockchains to ignite token use and fuel user adoption and experimentation.
Crypto faucets exist to introduce people to the concept of Bitcoin or cryptocurrencies. By providing tokens of no value (or occasionally, minimal value), users can interact with apps that require token ownership. Experimenting with a test transaction or app before using real money allows users to familiarize and test use cases.