When you breathe in your lungs expand and take in more air. When you blow up a balloon you can see the same effect.
Air pushes all the time in all directions.
It's called air pressure. Aeroplanes are reinforced to keep the air pressure constant in the passenger cabin. We know that air has an effect on fire because it spreads really rapidly when there is a lot of air, like when the wind blows, and fire dies when oxygen in the air is used up.