A cladogram is a visual representation of the best guess as to family groupings, or branches, within a subgroup. When the group is rather small it can look a little stringy, but as the group grows, blanks begin to get filled in and the branches become more and more clear.
Here is a brief guide to reading cladograms provided by Colleen Fitzpatrick**.
Yellow circles are haplotypes. The bigger the circle, the more people share this haplotype.
Red circles are median vectors. These are hypothetical people the software invents to knit together your members in the most efficient network. It's called the Maximum Parsimony Network. They represent either people who are not yet in your study, or people whose haplotypes have died out because they had only daughters.
The lengths of the lines between pairs of yellow or red circles are proportional to the number of markers that these haplotypes mismatch on.
The red labels on the lines are the names of markers that the mismatches occur on.
Don't stick on mv1 being everyone's common ancestor. This diagram shows you the relationship between the various branches of the family only. It doesn't necessarily say which one came first, or who their common ancestor was (though it might). The actual layout is not important – it's the geometry of the connections that is important.
** Colleen Fitzpatrick has two forensic genealogy websites, one featuring forensic genealogy information and another for her company Identifinders International. She also provides guidance on generating cladograms by various methods (scroll to #4).