Which one is correct,"what does it mean?or what does it means?"

The first sentence is correct.“what does it mean?”asks about something under discussion.

Gramatically if helping verb has the letter ‘s’ in the end,the following main verb has to be without ‘s’.thus what does it means ? is incorrect.

Generally no main verb following the helping verb takes ‘s’ with it.

Eg.

Does mean

Do mean

Shall mean

Will mean

Can mean

etc