"to import" is a transitive verb, i.e. it requires a direct object. And "to matter" is an intransitive verb, i.e. it doesn't require a direct object.
"Yo importo queso." (with "queso" being the direct object) would use the transitive verb, e.g. "I import cheese."
"Yo importo." has no direct object, so it is the intransitive verb, "matter", e.g. I matter.
I originally answered, "Cheese matters to me," but on reflection, that's not quite right. While "importar" means "to matter", it's a reflexive verb. Without a reflexive particle (like "me" or "se" or "te",) it's grammatically incorrect. Further, if that were the intention of the sentence, we would have seen "importar" conjugated as if queso were the subject, rather than the object; that is, it would've been "importa" rather than "importo".
So "Me importa queso" would, like various uses of "gustar", mean that cheese is important to me, but that's not the sentence shown here today. I was wrong.
I'm merely commenting on the possible confusion one might experience if they're not looking for the reflexive pronoun. That is, "Yo importo queso" and "Me importa queso" are superficially very similar sentences.
Specifically, I'm talking about the difference between "importar" and "importarse".