When most of us breathe, the muscles that are wrapped around the air tubes (airways) are very loose and relaxed, and the lining inside the airways are very thin. This lets the airways open up very wide so that it is easy to get air in and out of the small air sacs that make up your lungs. These small sacs are called alveoli. When air moves in and out, we call it breathing.
The picture below shows what your lungs look like when everything is working normally. The muscles that are wrapped around the airways are very thin and loose, and the airway is wide open. This makes it easy to move air in and out of the air sacs.