This is the 7931st most frequent Russian word.
классная
"Классная" means "cool" or "awesome" in informal speech, and can also mean "classroom teacher" in an academic context.
Here, 'классная' is used as an adjective to describe the quality of the book, implying that it is enjoyable or excellent.
Эта книга такая классная!
This book is so cool!
In this sentence, 'классная' is a part of the phrase that refers to a 'homeroom teacher,' a specific person associated with managing the class.
Моя классная руководительница устроила встречу.
My homeroom teacher organized a meeting.
Here, 'классная' is describing the cafeteria, suggesting it is stylish or superior in quality.