This is the 476th most frequent Spanish word.
primeras
"Primeras" means "first" (feminine plural).
Here 'primeras' is an adjective modifying 'flores', indicating their order in time (the first of the season).
Las primeras flores de la primavera son siempre hermosas.
The first flowers of spring are always beautiful.
'Primeras' modifies 'páginas', indicating their position as the first ones encountered in the book.
En las páginas primeras del libro se presenta al protagonista.
In the first pages of the book, the protagonist is introduced.
In this case, 'primeras' refers to the initial set of lessons, emphasizing the beginning of the sequence.