Someone is a word we use when we're talking about a person, but we either don't know who they are or don't need to say their specific name. When you hear a knock at the door and say “someone's here,” you mean a person has arrived, even though you can't see yet who it is. When your teacher announces that “someone left their lunch box in the classroom,” she knows a person forgot it but doesn't know which student.

The word works differently than anyone or everyone. If you say “someone ate the last cookie,” you mean one particular person did it (you just don't know who). But if you say “anyone could have eaten it,” you mean it's still a mystery with many possibilities.

We also use someone when the specific identity doesn't matter to the point we're making. A coach might say, “When someone on this team works hard, it inspires everyone else,” meaning any team member, not one specific player. Or you might observe that “someone who practices piano every day will improve,” talking about people in general rather than naming your friend Sarah who practices daily.

You'll sometimes hear people say that a person really is someone or became someone, meaning they achieved something impressive or important. But mostly, someone is just our everyday way of talking about a person without using their name.