Breathing is a component within the respiratory system, that is a complicated process in which air moves into or out from the lungs. The air that is exhaled is the air which is breathed out.
Exhaled air contains nitrogen, carbon dioxide, oxygen, and perhaps hundreds of additional chemicals.
When we exhale, the constitution of the air stays nearly the same as that of inhalation, with just the percentages of carbon dioxide and oxygen changing.
It is because the cells consume oxygen from the atmosphere to produce energy and emit carbon dioxide out as a byproduct.