なん meaning ‘what’ and used in different contexts.
Following on from what we already know; です and か putting those together with なん in front of them simply allows us to say “what is that/it/this?”
It is also often used as a substitute attached to other words to ask what they ask; for example, なんがつですか - what/ month/ is it/ ?
いちがつです - January/ it is
So by putting the ‘nan’ in place of the number of the month (ichi) - before the word for month, it is a simple way of asking the question.