When you talk about a plural, you're talking about more than one. The plural form of the word "goose" is "geese."
The word plural comes from the Old French plurel meaning "more than one," which is precisely how you would describe a plural form of something. In grammar, "the plural form" is used to describe more than one person, place or thing. Plurals can be tricky to master for people just learning English. There are the obvious ones where you just need to add an "s." Then, there are others: tooth, teeth; antenna, antennae; mouse, mice. The list goes on.