A noun is a person, place or thing as identified in a sentence. A possessive noun is a noun used in a sentence to show ownership. In the sentence "John's ball was lost the other day," the phrase "John's ball" indicates possession, and "John's" is the possessive noun. John owns the ball, another noun, that got lost.
The simplest type of possessive noun is a singular noun with an apostrophe and an "s" added to the end of the noun, such as in the phrase "John's ball." Plural possessive nouns are a bit trickier. In the sentence "The families' tents were blown over by the wind," the plural version of "family" indicates that multiple families had their tents blown over by the wind. In this instance, a single apostrophe is added at the end of the plural version of the noun, because the plural ends in "s." However, in the sentence "The men's clothes were too small," the plural noun "men" doesn't end in "s." In such cases, an apostrophe and an "s" are added to make the plural.