The graph view on the left shows your different development branches, for example. different sets of changes (possibly made by different people):
Person A makes revisions 17, 20, and 25.
Person B (or the rest of the team) made other changes ... and at some point the changes should be merged again.
The different colors of the lines are for clarity only, and the colors are randomly assigned, I think.
(see TortoiseHG docs for a screenshot with lots of lines )
In your example, the blue and red line connecting in revision 26 means that you (or someone else) have combined revisions 24 and 25 (the blue dots where the lines come from).
The same thing happened in edition 24, where versions 20 and 23 were changed.
The fact that you are asking about this shows me that the concept of branching and merging in Mercurial (or any VCS) seems new to you.
Perhaps you should read a little about branching and merging. Here are some good lessons: