This is the 14804th most frequent Russian word.
плохая
"Bad" or "poor" (feminine).
The word "плохая" is used here as an adjective in the nominative case, describing the quality of the book, which is the subject of the sentence.
Эта книга плохая.
This book is bad.
In this sentence, "плохая" is used as an adjective to describe "девочка" in the nominative case, within a simile construction.
Ты ведёшь себя как плохая девочка.
You behave like a bad girl.
Here, "плохая" describes "погода" as an attribute, agreeing in case, gender, and number with the noun it modifies.