They and their are gender-neutral pronouns preferred by many non-binary individuals. These pronouns are used in preference to the pronouns typically associated with an individual’s gender assigned at birth, e.g., he and his, she and hers.
They and their, when used as inclusive pronouns, help fill the gap in English language since it does not have any non-binary, third-person pronouns as some languages do. Incorporating these terms into common usage recognizes gender identities outside the binary genders.