An outsider is a stranger — someone who doesn't fit in, or someone who observes a group from afar. An outsider stands outside the group, looking in.
If you go through high school without belonging to any particular group — you're not a jock, a nerd, or an artist, for example — you might feel like an outsider. Having a brand new job can also put you in the position of being an outsider, especially if you have no experience in the profession. To an outsider, the way a busy restaurant works might seem chaotic and random, for example. To an insider, on the other hand, it might all make perfect sense.