During the Primary Survey, you will determine whether or not your casualty is unresponsive but breathing (unconscious) or unresponsive and NOT breathing, in which case they are dead and you will need to undertake CPR.
Why do we do this ?
In order to preserve life, we need to maintain a constant flow of oxygenated blood to the organs and tissues of the body, and this is what CPR does.
"Cardio Pulminary Resuscitation" has clues in the name !
If you see the word cardio, it means that we are talking about something relating to the heart.
If you see the word pulminary, it means we are talking about something relating to the lungs.
And the word resuscitation literally means "returning something to a previous state."
So, in simplistic terms, when we talk about Cardio Pulminary Resuscitation, we are talking about a process whereby we are trying to return the casualty to a healthy state, by manually operating the heart and lungs until such times as the casualty is able to do so for themselves.
In truth, though, all we are really doing is circulating oxygen to the deprived organs and tissues in the body so that, at cellular level, they don't die off.
By breathing in to the casualties lungs, we provide them with oxygen which is transferred into the bloodstream, and by pumping the heart( by pushing down on their chest) we force that oxygen enriched blood around the body, 'feeding' the organs and tissues.
Essentially, we are keeping them 'fresh' until the heart can be restarted, and they start breathing for themselves.