The Brief: They, them, their, and theirs are gender-neutral third person pronouns.
They, Them, Their
They, them, their, and theirs are gender-neutral pronouns that can be used to refer to people who are non-binary, genderfluid, genderqueer, trans, and/or don’t identify with the gender binary.
More Gender-Neutral Pronouns
She/her/hers and he/him/his are pronouns for people who identify along the gender binary, as men or women. They/them/their are one set of gender-neutral pronouns. Other commonly used gender-neutral pronouns include Ze/zir/zem/zeir, Xe/Xir/Xem/Xeir, and hir/hirself. Trans people sometimes use binary or non-binary pronouns, depending on what their gender is and how they identify.
The best way to avoid using the wrong pronouns to refer to someone is to ask them which ones they use and to let them know which ones you use.