This is the 454th most frequent French word.
français
"Français" means "French" in English.
Here, "français" refers to the French language, used as a noun.
Je parle français couramment.
I speak French fluently.
Here, "français" specifies the language in which the content (the book) is written, functioning as an adjective.
Nous avons un livre écrit en français.
We have a book written in French.
In this example, "Français" is used as a noun to refer to the French people.