Your heart is an important muscle that pumps blood throughout your body. In that blood is oxygen and other things like plasma, which carries nutrients and hormones, platelets, which block the site of an area where you have an exposed cut, white bloodcells which fight diseases, and red blood cells which carry oxygen. It carries all these things to most of your organs, and most importantly your lungs. The heart takes blood that does not have oxygen and pumps it through your veins to the lungs so they can oxygenate the blood before it circulates throughout the arteries of your body.
In your heart there are two "pumps" that have to work together in order to keep you alive. Two things happen in the right and left side of your heart. On the right side, blood that is flowing back from your organs and tissues comes into the right side, and is then pumped into your lungs for oxygenation. On the left side of your heart blood that has already been oxygenated is circulated back from your lungs and goes into the left side of your heart which displaces it around your body, nourishing it and making sure that your body has enough nutrients and oxygen.
Like how the heart has a left and right side, it also as four chambers that are divided equally between left and right. The two chambers that are located at the top of your heart are called atria, and below are called ventricles, which have on valve at the exit and entrance so blood doesn't flow the wrong way.
The atria and ventricles have "on and off beats" that work together to excrete blood out of the heart and filling it with new blood. An electrical signal near at the top of your heart signals it to contract, the atria contracts first and excretes the blood through the open valve and into the ventricle. After that the electrical signals move into the ventricle and again makes it contract and excrete blood to your lungs and the rest of your body. When the atria and ventricle de-contract, or relax, that allows them to fill with blood because they are not pushing it out.