Someone who's standoffish is aloof or unfriendly. When you first meet someone, she might seem standoffish when really she's just shy.
A politician who appears standoffish will have more trouble getting elected than one who seems warm and likable. The adjective standoffish is a great informal word for describing people who are reserved or haughty or cold. Standoffish comes from a now obsolete meaning of the phrase stand off, "hold aloof."