I have seen these icons coming in front of sms messages and in contact list. What they stand for?
These icons are just like a "clock"
The icon refers to that contact's "availability":
- Green: Available. You are online and ready to chat.
- Yellow: Idle. Your status automatically changes to idle when you're away from your computer for 15 minutes. You can't manually select to appear idle
- Red: Busy. This lets your contacts know you're busy and don't want to be interrupted
- Grey: Signed out of chat, or Invisible. If you set your status to Invisible, you'll appear to be signed out of chat even when you're actually signed in.
Source: Google Chat support answer