A noun is a name we give to things, places, people, animals, emotions, etc. so we can identify them. Nouns and noun phrases answer the questions what? or who? They may be:
- the subject of a verb
Her cousin in Tokyo called her yesterday.
- the direct object of a verb
My boyfriend took an incredible photo of a deer drinking from a stream.
- the indirect object of a verb
I gave my son a present.
- the object of a preposition
I heard about it on the radio
- the complement of be
Mr. Smith is their neighbor
- used "in apposition" which means "a grammatical construction in which two usually adjacent nouns having the same referent stand in the same syntactical relation to the rest of a sentence" (Merriam-webster.com)
Joshua Perry, a Google engineer, explained the complicated issue
- used when addressing somebody directly in speech
Elizabeth, lock the door, will you please?