stranger
A person you do not know yet.
A stranger is someone you don't know. The person sitting next to you on the first day of school is a stranger until you introduce yourselves and start talking. The family that just moved in down the street are strangers until you meet them.
Parents teach young children to be careful around strangers for safety reasons, but it's worth understanding what this really means. A stranger isn't automatically dangerous, they're simply unfamiliar. Most strangers are perfectly ordinary people going about their day. The challenge is that you can't tell just by looking whether someone is trustworthy, which is why adults remind kids to stay close in public places and not to go anywhere with people they don't know.
As you get older, you'll learn to navigate a world full of strangers: the cashier at the grocery store, the librarian helping you find a book, and the other families at the park. Every friend you have was once a stranger.