Look at the front of your car, or any car for that matter. You see that opening (the grill) in the front? The grill allows air from outside to come into the engine and combine with the car’s gasoline to produce energy.
In the world of fitness, we like to use the analogy of your body being like “your car” because, in many ways, they perform in the same manner. Your primary grill is your nose, and depending on other factors, your mouth becomes a secondary grill.
So, when doing cardio for weight loss or for greater aerobic endurance and strength, (in other words to run farther for example), one must consider two things: 1) how do we get in enough oxygen, and 2) how do we get our bodies to use this oxygen most effectively to burn more fat? This is called your oxygen capacity.