## 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,