Who Is Surprised?
A surprise round occurs if some but not all characters involved in a combat are unaware of opponents about to attack. Sometimes all the combatants on a side are aware of their opponents, sometimes none are, and sometimes only a few are. Sometimes several combatants on each side are aware while the rest are surprised. When you can sneak up and catch your enemies by surprise, you enjoy a powerful advantage. It’s one way stealthy characters can even the odds when facing foes with superior combat abilities.
Source
Core Rulebook pg. 239