How to explain $log (P(X))=sum_zQ(zvert X)log (P(X))$?

How to explain

$$log (P(X))=sum_zQ(zvert X)log (P(X))$$

here $P(X)$ is a probability of $X$, and $Q(z|X)$ is conditional probability. I don’t understand this equation. Any explaination?

Thanks,