I think it shows which revision is included in a tag or branch you've created. For example I have a file with revisions 3*, 10, 13, 32*, 34* and I have created 3 tags from my trunk. These tags were created with revision 8, 33 and 35.
So revision 3 is included in my tag I've created with revision 8, revision 32 is included in my tag with revision 33 etc.
Revisions 10 and 13 are not marked with * because the higher revision 32 was tagged.
Due to this fact, I think that it is related to the creation of tags and/or branches.
I hope you understand my explanations ...