Oxygen and Carbon Dioxide both the gases are a part of Air we breathe in.
The air we intake is a mixture of Nitrogen, oxygen and carbon dioxide. So, this air when enclosed in a cavity would exert a total pressure. However, the total pressure generated by the air is due in part to nitrogen, in part to oxygen, & in part to carbon dioxide. That part of the total pressure generated by oxygen is the 'partial pressure' of oxygen, while that generated by carbon dioxide is the 'partial pressure' of carbon dioxide. A gas's partial pressure, therefore, is a measure of how much of that gas is present (e.g., in the blood or alveoli).