This is the 11118th most frequent Polish word.
generałów
"Generałów" is the genitive plural form of "generał," meaning "generals" in English.
Here 'generałów' is in the genitive plural form, following the preposition 'o', indicating what the book is about.
Książka o życiu generałów była pasjonująca.
The book about the lives of generals was fascinating.
The word 'generałów' is used here in the genitive plural form to denote 'of generals', used after 'portretów', specifying whose portraits are being referred to.
W muzeum można zobaczyć wiele portretów polskich generałów.
In the museum, you can see many portraits of Polish generals.
In this sentence, 'generałów' is again in the genitive plural form, governed by the numeral 'dwóch', modifying its role in the sentence as the object receiving the action.