What are pockets exactly?

The sinuses are four pairs of air-filled cavities in the bone of the skull: the sinuses of the front of the head (above the eyes), the sinuses of the cheekbones, the sinuses at the root of the nose (between the eye sockets), and the sinuses of the sphenoid bone behind the nasal cavity and eyes.
Mucous membranes line the sinus cavities, and are connected to the nasal cavity with small openings. The sinuses, when well functioning, contribute to warming and humidifying the air passing through the nose. The sinuses work with the nasal cavity as a kind of bony cushion protecting the face and brain. They also have advanced benefits. The sinuses add depth and resonance to the voice.