[Math] What does 2 values vertically arranged in parenthesis in an equation mean

Question

Below is the particular expression I am concerned with.

I suppose it denotes some sort of operation but I am unsure as to what. I'd appreciate some guidance. Thanks.

Answer 1

It is the Binomial coefficient $$ \binom w i = \frac{w!}{i!(w-i)!} $$ Or $$ \binom w i = \frac{w^{\underline i}}{i!} $$ where $w^{\underline i} = w(w-1)\dots(w-i+1)$ is the falling factorial.