Should I say "what does it means" or "what does it mean"?
Here are the relevant rules:
A clause never has more than one finite verb - a verb that is marked for tense, person and number (to the extent that it can be marked for any of these categories.
When the verb (predicator) in a finite clause is constructed with auxiliaries (helping verbs), the finite verb is always the first auxiliary in the chain.
Each auxiliary verb licenses specific non-finite forms (present or past participle or infinitive) for the following non-finite. For instance BE licenses either a present or past participle, HAVE licenses a
past participle, and so forth. Auxiliary DO licenses only an infinitive.
In questions, the subject and the first, finite auxiliary switch places; if there is no auxiliary, the ‘dummy’ auxiliary DO is introduced (this is called *DO-support'), and takes the appropriate finite form.
In your question, DO-support is invoked to provide an invertible finite auxiliary, does. It must be followed (after the subject) by the infinitive form of the lexical verb MEAN.
In your answer, DO-support is not required; MEAN is the only verb and takes the finite form which bears the desired tense (present) and agrees with the subject in person (3d) and number (singular)—means .