Note: If you say in Arabic “This is a door” it will be “Hadha baab” or “Hadha baabun“.
what is the difference between “baab” and “baabun“?
The “un” in the end of the noun we call it “Tanween” in Arabic, and it is a sign for indefinite nouns, like in English “a, an”, a door, so the both words “baab, baabun” are correct.