A noun clause is a group of words that functions as a noun in a sentence. It can act as a subject, object, or complement. For example, in the sentence "What she said surprised everyone," the clause "What she said" serves as the subject of the verb "surprised." Noun clauses often begin with words like that, what, who, whom, which, or whether. They can provide essential information or complete the meaning of a sentence. For instance, in "I believe that he will win," the clause "that he will win" acts as the object of the verb "believe."