Our hearts pump our blood through our blood vessels through our bodies and also to our brains.
Blood is important to our brains and if our brains don't get enough blood and the blood going to our brains is rich in oxygen which our brains need to stay healthy and survive.
When our brains don't get that oxygen rich blood then the brain cells can start to die off which can lead to brain death and even brain damage should the person survive a heart stopping.
So it's important to get the heart beating as soon as possible if you can because the person could die soon after the heart stops or the person could have brain damage as a result.