This is the 9176th most frequent Portuguese word.
caber
"To fit" or "to be suitable."
In this context, 'caber' is used figuratively to indicate responsibility or obligation associated with the subject.
Cabe a nós tomar a decisão final.
It is up to us to make the final decision.
In this sentence, 'caber' is applied to express the appropriateness or relevance of an action considering the circumstances.
Não cabe discutir isso agora.
It is not appropriate to discuss this now.
Here, 'caber' is used to express that the book's dimensions are too large relative to the space provided by the backpack.