What is a plural noun

Consider a noun such as BOY. When there is only one (a single one) you can say something about the boy, or about a boy.
So you use the form BOY when there is a single one. This form is called the singular form of the noun BOY.
However if you are speaking about or pointing at a group ( of people ) in which everyone is a boy you use the modified form BOYS. This form is called the plural form of the noun BOY. The word plural means many.
The plural of a noun is usually made by adding s to the singular. It is pronounced S after a p, k, or f sound. Otherwise it is pronounced Z.
There are certain exceptions to this rule, the ones that come to my mind are: one child, three children; the man, 10 men; one woman, many women; a single knife, 12 knives; a potato, several potatoes; the top shelf, all the shelves.
Note: When you look for the meaning of a noun in a dictionary, you look for the singular form, although the plural is indicated in full if it is irregular (does not follow the rule).