A noun clause is a clause that is used in the same way as a noun or a pronoun.
I know what he was doing there.
In the classroom
A way for learners to identify a noun clause is for them to change the noun clause for ‘it', ‘he' or ‘she' and see if it still works. Noun clauses are a useful area to explore with more advanced learners, who can make their language more sophisticated by extending the complexity of noun clauses they use.