What does the "0 ≤ K < N-1" expression means?
You have an array with indices from $0$ to $N-1$. $K$ is an in integer in this range, i.e. larger or equal to zero and smaller than $N-1$.

This integer splits the array into two parts as follows:

Part 1: From $0$ to $K$ (including K)

Part 2: From $K$ (excluding K, i.e. starting with $K+1$) to $N-1$. 

$K$ has to be smaller than $N-1$ (and not smaller or equal) because else there would only be one Part (the whole array) for $K= N-1$.