Some more context, like the root question would make it easier for people to give you a correct answer. But my guess would be the pause between the expiration and the next inspiration, or vice versa.
When the chest is at rest you are in the relaxed state. During relaxation of the chest your intercostals are "pressing" against the lungs and causing exhalation of CO2 and other bodily gases. You are increasing pressure on your lungs which in turn exhales a volume (the tidal volume) of air. When you inhale, you are ACTIVELY expanding your chest, your intercostals retract your diaphragm and your lungs experience a decrease in pressure which will in turn cause the outside air to move into your lungs to reach pressure equilibrium. Hope this helps.