They is a pronoun that is most commonly used as the plural of "he," "she," or "it."
Throughout history the word has been used to refer to the plural form of he/she/it, but also in a singular form when gender is either unknown or unspecified. This has become more pronounced in recent times, with the feminist movement desiring the use of the singular "they" in place of "he" or "she" when gender is unspecified, with the aim of ending perceived bias. Conservatives view this as a politically correct alternative pushed by liberals, which is often redundant as "he" has traditionally been used to refer to both men and women.
Prescriptive grammarians also usually object to the singular "they", saying that it detracts from grammatical regularity and clarity. However, this opinion ignores the fact that, in the second person, the plural "you" replaced the singular "thou" centuries ago with no real ill effects.