Question

what does "k" stand for?

Answer 1

a variable

Answer 2

If you mean k as the variable in probability/statistics , then it stands as the probability of getting exactly k successes in n trials from
\[\huge \left(\begin{matrix} n \\ k\end{matrix}\right) = \frac{ n! }{ k!(n-k)!}\]

Answer 3

k is a variable.