Perception score is the sum of a creatures attention, sharpness of senses and the general awareness of their surroundings.
Creatures with high Perception are able to detect incoming threats quickly, more easily spot something hidden in cluttered environments and in general use their senses to detect even minute details with their senses. Low Perception represents creatures may lack peripheral vision, are slow to act in high pressure situations and are in general oblivious most minor sensations, like the intricacies of a dish.
Passive Intuition
You add you Perception modifier to your passive intuition. Passive intuition represents your perception of your surroundings and general intuition in new situations and areas while exploring. More can be read about it in the rules of adventure chapter.
Perceptive Initiative
You add your Perception modifier when you roll initiative at the beginning of every combat. Initiative determines the order of creatures' turns in combat, as described in the Combat chapter.