In contemporary English, people have generally been referred to by one of two sets of pronouns: “she/her/hers” for women, and “he/him/him” for men. Generally when someone meets a new person, they make a snap judgement about whether that person is a man or a woman and use the corresponding pronouns.
Not everyone looks conventionally masculine or feminine - and some people are neither! People who are transgender, nonbinary, or just don’t particularly conform to gendered stereotypes all upend our ability to make quick decisions about what gender someone is. Not everybody uses “he/him” or “she/her” pronouns. Because those are both so heavily tied to two rigidly defined genders, some people prefer to express their sense of gender by using pronouns that aren’t tied to any particular gender. "They” has been used as an ungendered pronoun in English since at least the 14th century, and is the most commonly genderless pronoun.
Some people get confused by the prospect of using “they” as a pronoun, and it can require a bit of practice to get used to. Some people prefer other, more recently invented pronouns as a break from gender. These are sometimes called “neo-pronouns,” and some examples may include xe/xem/xyr, e/em/eir, and ve/ver/vis.