Here's why:
* French is a gendered language. This means that nouns and adjectives are assigned a gender, either masculine or feminine.
* "Féminin" is the French word for feminine, and it's also a feminine noun.
* "Masculin" is the French word for masculine, and it's also a masculine noun.
So, both "féminin" and "masculin" are adjectives and nouns that represent the genders in French.