Sinusitis is an inflammation or swelling of the tissue lining the sinuses. Sinuses are hollow spaces within the bones between your eyes, behind your cheekbones, and in your forehead. They make mucus, which keeps the inside of your nose moist. That, in turn, helps protect against dust, allergens, and pollutants. Healthy sinuses are filled with air. But when they become blocked and filled with fluid, germs can grow and cause an infection.