This is the 15266th most frequent Russian word.
посерьезнее
"More serious" or "a bit more serious."
'Посерьезнее' is used comparatively here to indicate that the subject of discussion should be more serious than other topics previously mentioned.
Давай поговорим о чем-то посерьезнее.
Let's talk about something more serious.
'Посерьезнее' is used to specify a need for a more serious attitude or behavior regarding responsibilities.
Ему нужно стать немного посерьезнее по отношению к своим обязанностям.
He needs to become a little more serious towards his responsibilities.
'Посерьезнее' describes the book, suggesting its content should be of a more serious and profound nature for detailed study.