Pronouns indicate the gender of a person; traditionally, he refers to males while she refers to females. The English language does not have a gender-neutral third-person singular personal pronoun, but in recent years they has gained considerable traction in this role. They has been officially recognized as correct by several key bodies such as the Associated Press. Similarly, the Chicago Manual of Style now notes that the singular "they" is common in informal communication (while acknowledging that it has yet to attain the same ubiquity in formal spaces).
Knowing that "they" can be used to refer to individual people allows writers to avoid defaulting to he in regular use. It is also important for people whose genders are neither male nor female.
https://owl.purdue.edu/owl/general_writing/grammar/pronouns/gendered_pronouns_and_singular_they.html