The large bracket is this:
)
(yes, it's a bracket but bolded.)
Say, you have a function, i.e.
b(n,m) = b(n-1,b(n-1,b(n-1,b(n-1,m))))
Isn't that string of brackets at the end clumsy?
The large bracket is useful now.
b(n,m) = b(n-1,b(n-1,b(n-1,b(n-1,m)
This way, we can close off functions which need tw inputs to function, as recursion doesn't work in this scenario.