Notation: for strongly-regular graph $(v,k;\lambda, \mu)$, how to interpret "$k(k-\lambda-1)$" and "$\mu(v-k-1)$"?

Question

Proposition: Let $G=(V,E)$ denote a $(v,k;\lambda, \mu)$ strongly-regular graph. Then $$k(k-\lambda-1)=\mu(v-k-1)$$

The notation is quite overloaded for me. Since $k$ and $\mu$ are parameters, then what does $k(k-\lambda-1)$ and $\mu(v-k-1)$ mean?