The faucet allows you to get some "test ada", so that you can try out features that require funds.
In order to use the faucet, you first need a payment address, which can optionally be associated with a stake address. You can get it with Daedalus STN or with the CLI (See Generating payment/staking keys and address)
Access the faucet here and follow on-screen instructions
Each time you use the faucet (which you can do once a day), you will get 100000 testnet-ada.