When odorants in the air bind to the olfactory receptors attached to the glomeruli in the nasal cavity, a threshold is attained. A signal is generated and travels through the olfactory nerve to the olfactory bulb in the brain.
The signal is interpreted by the brain to stoke emotions, memory and behavior. Odorant that triggers the olfactory system include pheromones, food, hazards and etcetera.