The ‘*’ mark shows which revision is included in a tag or branch you've created. For example a file with revisions 3*, 10, 13, 32*, 34* and have created 3 tags from the trunk. These tags were created with revision 8, 33 and 35.
So revision 3 is included in the tag being created with revision 8, revision 32 is included in tag with revision 33 etc.
Revisions 10 and 13 are not marked with * because the higher revision 32 was tagged.
Due to this fact, it is related to the creation of tags and/or branches.
The graph does not show tags as it would be too noisy. So it puts an * next to items that are tagged, and the tooltip lists the tags.