Log: explain meaning of branch refs (tracking vs. non-tracking), e.g. using tooltips

Please, tell me, where can I find informations about icons meaning below?

In first picture, origin and local develop branches are "connected".

In second picture, icons are not connected.

Head is the same, diffs reporting that files are 100% identical... but in one case is icon "connected" in other one are "disconnected". What does it mean? :)

Kindly thank you for your reply.