Learn Spanish (Latin America): No nos importa la acción.

Importar works just like gustar. Instead of saying "We don't care about the action," Spanish phrases it backwards: "The action is not important to us" (or "doesn't matter to us"). Because the subject doing the verb is actually la acción (a singular noun), the verb is conjugated as importa. The pronoun nos (to us) just tells us who it matters to.

The English translation says "care about". Why is there no word for "about" in the Spanish sentence?

Because of how importar works, la acción isn't an object we are caring about; it is the subject of the Spanish sentence. Literally, you are saying "The action doesn't matter to us." Since there is no "about" in the Spanish structure, you don't need to add a preposition like de or sobre.

Why does no come before nos? Can I say "Nos no importa"?

No, you can't put no between the pronoun and the verb. In Spanish, object pronouns like nos usually need to stick right next to the conjugated verb (importa). Because they are glued together, the negative word no has to go at the very beginning: No nos importa.

Is acción always feminine?