Partitive plural is a grammatical number that is used to modify a noun which represents a part of some whole amount, as opposed to the comprehensive plural, used when the noun represents the total amount of something. This plural form is used in the Finnish language, and its use in this language influenced J.R.R. Tolkien in inventing his fictional language Quenya. It refers to more than one object that is nevertheless only part of a larger number; for example, three out of four of something. It is one of four grammatical numbers in Quenya, the others being singular, dual, and plural.
As for the Finnish partitive plural, it is really a regular case form in the plural number, but which is used in the singular as well, denoting an abstract, unknown identity or result. The best example of this would be the Finnish word for snow, "lumi". The partitive plural is "lumia", expressing the concept of more than a snowflake (lumihiutale) but not all snow everywhere (lumi).