They is a pronoun we use to refer to more than one person, or to refer to a single person whose gender we don't know or when gender doesn't matter.

When talking about multiple people, they is straightforward: “My friends said they would meet me at the park.” But they also works perfectly well for one person. If you find a lost backpack, you might say, “Someone left their backpack here. I hope they come back for it.” You're not saying the owner is multiple people; you simply don't know the owner's gender, so they works naturally.

Some people also ask to be called they instead of he or she because that feels more accurate to them.

They is also useful when you're talking about people in general: “They say it might rain tomorrow” or “You know what they say about practice making perfect.” In these cases, they refers to people broadly, without naming anyone specific.