You can think of any Wolfram Language expression as a tree. In the expression above, the top node in the tree consists of a Plus. From this node come two "branches", and . From the node, there are then two branches, and , which can be viewed as "leaves" of the tree.
The indices that label each part of an expression have a simple interpretation in terms of trees. Descending from the top node of the tree, each index specifies which branch to take in order to reach the part you want.