# Clarification on descending direction in optimization of function

Could someone clarify for me why given $$f:\mathbb{R}^n \rightarrow\mathbb{R}$$ to optimize an iterative function according to :

$$p^k=-M\nabla f(x^k)$$

for $$p^k$$ to be descending direction the matrix M has to be positive definite?

Feel free to explain it as simple as possible. Thanks in advance.