I think you mean the green dot that indicates whether an army is locked or not.
If such a dot is in place, this army is locked, which means it's grouped together / considered as one unit.
You can lock / unlock armies by clicking on the small red button under the big green button that takes you to your city view.

Quote owdsyko:

these armies can not be selected fo campaigns

You should be able to send locked armies on campaigns!
That's the purpose of this feature:
Being able to form locked groups whose settings will be preserved. A list of the locked armies should be available under the place where you select troops from the non-locked armies.

Problem solved? Any more questions on this? -> Post again in this thread.