Let's see how we can give these words articles. An article is something like, 'the' or 'a'.
Definite article, 'the'. The car.
Indefinite article: 'a'. A car.
So, let's do it in Spanish.
Un libro/A book. Masculine.
Un auto/A car.
Let's repeat: Un libro. Un auto.
Now, let's do the feminine.
Una casa/A house.
Let's repeat: Una casa.
Una moneda/A coin.
Let's repeat: Una moneda.
A female cat is una gata. A male cat is un gato.
Does this make sense to you? If perro is dog, how would you say the female dog? That's right. Perra.