This is a cladogram.
Cladograms show evolutionary change over time. Time moves from left to right along the main line at the bottom. The branches off that main line show the evolution of different organisms. Organisms to the left of a branch share one or more similar feature(s) that evolved over time. If a branch splits into smaller branches (such as the one that splits into reptiles and birds), that means those organisms are more closely related than to the other branches (like in a family tree!)