Two other items here are worth your attention. The first is the FaceTime On/Off toggle. If you don't wish to be reached over FaceTime on your computer (you prefer, for example, to conduct such chats on your iPhone), just flip the toggle to Off.
The second is the Apple ID field. Here you'll find the address associated with your Apple ID. Click it to see options for changing your location and viewing your account. You'd change your location so that if you and your Mac are in another country and you have a contact within that country whom you'd like to call, you can do so via a local call rather than one placed overseas. (This isn't a cost-cutting measure, but rather a way to tell FaceTime not to bother dialing a country code.)
When you click View Account and then enter your Apple ID's password, you can sign in to your account and change your name and password. If you click the Manage Apple ID link, your default Web browser will open; there you can enter your Apple ID and password, and then go to a page for managing elements of your account (such as the credit card associated with it).
In the pane where Change Location and View Account appear, you also have the option to sign out of your account. Do so, and you return to the Sign In screen you saw when you first launched FaceTime.
This screen is useful for more than signing in to your usual Apple ID. If you have more than one Apple ID, or if you're using a friend's or family member's Mac, you can sign out of the usual account and sign in with another. Although this requires more steps, it's a bit like using someone else's phone to make a call.
And that's the miracle of FaceTime—a way to make video calls without spending an additional nickel.
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