The image below is a phylogram, illustrating the degree of variation in a sequence of mitochondrial DNA. The concept is fairly simple: if two DNA samples are from individuals that are evolutionarily distant from one another, they’ll have accumulated more differences in their mitochondrial DNA, and will be drawn farther apart from one another. If the two individuals are closely related, their DNA will be more similar, and they’ll be drawn closer together. That’s the key thing you need to know to understand what’s going on.
There are other, more complicated analyses going on in the figure, too: the branching pattern is determined by analyzing subsets of shared sequences, and it takes a fair bit of computing power to put the full picture together. You’ll just have to trust me on that one, but all you need to know is that the branches are objectively calculated, and that the distances between the tips of the branches and their last branching point tell you something about the degree of genetic disparity in the group.