I'm having trouble understanding a function, in relation to binary rooted trees

Def'n: abinaryrootedtree is a tree with a root node in which every node has at most two children.Let

Def'n: A terminal is a node with no children

Let T be a tree, B the set of all BRTs, n(T) counts the number of nodes in T, z(T) counts the number of terminals

Noting the recursive structure of BRTs, let L be the left subtree, R the right subtree

if either L or R not empty, 1 if both are empty

Thus (this step I don't understand)

I'm having trouble understanding how the function is broken down.

Help please?