Changes graph, how does it actually work?

I am using mercurial with Webstorm and as you know you can see the changes graph in the changes tab. But I am having a real hard time trying to understand what I am seeing. Does anyone know of a document actually describing how to read the tree? The help page for this tab doesn't really help for this.

(And where are the revision numbers hidden? All I see are these long hashes).

Thanks in advance,
Danny