The case of a noun (or pronoun) can dictate how it can be used in a phrase or clause.  Three common cases of nouns are:  Subject, Object, Possessive

Subject case:  this is when the noun stands alone.  For example, Mark is going to the store.  Mark is the noun in subject case.

Object Case:  this is when the noun is used for the object of a verb.  For example, Mark is driving the red car.  Car is noun in object case.

Possessive case:  this is when any noun acts as an adjective, thus modifying another element in the same sentence.  For example, the dog’s hair is wet after a shower.  Dog is the noun with a possessive case.