Vignesh has the right answer, but just to add, this is not Anaplan specific. Anaplan is making use of the 'Tree structure' terminology to make a visual representation of the data hierarchy, explained on wikipedia here: https://en.wikipedia.org/wiki/Tree_structure
The general idea is that the highest level (e.g. All Products) is the root of the tree, then each level below it are the tree branches and the last level are the tree leaves. With this representation in mind, it is easier to understand that you can go from a certain leaf to its branch to the next branch all the way to the root and that from the root you are able to reach all leaves.
PS. It is a bit unfortunate that the picture is usually shown as vignesh did where the root is at the top and the leaves are at the bottom, leaving the tree upside down.