(I don't think we can really say that an NP is equal to a noun because nouns represent a category on a lower level than a noun phrase.
A noun phrase always contains a determiner (whether visible or not) and a noun. Sometimes a noun phrase can look like it contains only a noun but we then assume that there is a zero determiner before that noun that functions as a head word of that noun phrase.
This is the case when we have uncountable nouns or nouns in plural. For example, "I drink [NPcoffee] every morning."
The noun phrase "coffee" consists of a zero determiner, and a head noun "coffee".)
What is meant by zero determiner and the nature of it being visible or not?
Where would it be in this example: The students in the class listened carefully to the teacher
Head word is students, right.