The function of a word is what job it is performing in the sentence. A noun generally functions in one of three ways:
These three are what we call “functions”. To identify a given word as a noun is to tell what that word is (or its part of speech). To identify a given noun as a subject is to tell what that noun does (or its function). Merely to identify a word as a “noun” does not tell what its “function” is.
Nouns can also function adjectivally or be in apposition to another noun. See this video or the presentation below.