Seeing if someone is online and available to talk
A red ball next to someone's name lets you know that person has designated themself as Busy.
If one of your friends is away or inactive, you'll see a yellow ball next to his or her name.
And when your friends are offline, a gray ball will appear next to their names to let you know they aren't signed in to Google Talk.
No ball next to someone's name? Invite your friend to Google Talk.