A stranger is someone you don't know or who doesn't belong in a specific place.
Parents tell their kids, "Don't talk to strangers." That's because strangers are people they don't know, who could be dangerous. Just like strange things are odd or weird, a stranger is unknown and therefore potentially scary. Also, anyone out of place can be considered a stranger. A fireman in a police station is a stranger, and a mascot in a scientific lab is a stranger. Either type of stranger is likely to make someone say, "Who's that?"