The bubble shows you when the user was online:
green = user is online
green/yellow = the user was within the last hour online
yellow = the user was within the last 24 hours online
grey = the user was not online within the last 24 hours
If you have a prime subscription you can also see the exact online time
English version: Der grüne, grün/gelbe, gelbe und graue Punkt bei dem Profilbildern