The function of the noun clause in this sentence is a subject.
A noun clause is a group of words working together as a noun. These clauses are always dependent clauses so that, they do not form a complete sentence. Noun clauses are used to name something when a single word isn't enough. These can be noticed by seeing the following words: How, that, what, who, why, whoever, whatever, when, where, wether, which, whichever, who, whoever, whom.