What is the intuition behind this?
Let $Y=zF(K,N)$ be a production function. For a profit to exist Indana conditions must hold and :
$$\frac{∂^2zF(K,N^d)}{\partial K \partial N} > 0$$
I understand all the conditions intuitionally, but I can understand this one mathematically but can't understand even what is meant by $\frac{∂^2zF(K,N^d)}{\partial K \partial N} > 0$. What does this being greater than zero imply?