What does the green dot on Hangouts mean?

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Tags

1. #messages
2. #invitations
3. #picture
4. #immediately
5. #turned
6. #about
7. #setting
8. #hangouts
9. #someone
10. #notified
11. #these
12. #people
13. #green
14. #classic
15. #available
16. #video
17. #profile
18. #their