The image shows what is called a cladogram. It is a tree-like structure which represent evolutionary relationships. It starts with a single root and splits into two taxa which themselves might split. From the root the first to split of is Petromyzontoidea, then Elasmobranchii, then Actinopteryaii, then Dipnoi, and then Amphibia. The remaining taxa splits into two groups: the Mammalia and a second taxon which splits into Testudinata and another taxon. That taxon first has Lepidosauria split off and then splits into Crocodilia and Aves.