This is the 3342nd most frequent Russian word.
плохой
Bad.
Here, "плохой" is used as an adjective to describe the quality of the book, implying that it is not good or desirable.
Эта книга плохая.
This book is bad.
In this case, "плохой" is used to describe the abstract concept of one's mood, indicating it is negative or unpleasant.
У меня плохое настроение.
I am in a bad mood.
Here, "плохой" is used to characterize the weather as undesirable or unfavorable.