A noun clause functions like a noun in
a sentence. Noun clauses are introduced by subordinating words such as what, that,
when, why, how, whatever, who, whom, whoever, whomever. (A clause is a noun clause
if you can replace the whole clause with the pronoun something or someone.)
He wants to know what he should do.
[noun clause = something: what he should do]
Whoever scores a goal will be a hero.
[noun clause = someone: Whoever scores a goal]