To get oxygen and nutrients to the cells in your body, and to get rid of waste, your blood needs to keep flowing constantly. The pumping by your heart makes sure that it does. This is what we call blood circulation.
Your heart is made up of two halves, with two ventricles, two atria, and four heart valves. The left and the right half of the heart are separated by two walls, the atrial septum and the ventricular septum. The contractions of the heart muscle make the ventricles pump the blood in their respective directions.

This means that you have two separate blood circulations. The high-oxygen blood and the low-oxygen blood each have their own circulation and cannot mix.

The right ventricle pumps blood into the pulmonary artery (small blood circulation) and the left ventricle pumps blood into the aorta (large blood circulation).
The animation shows exactly how your blood flows through your heart.
From the left ventricle, the high-oxygen blood flows into the large artery (the aorta). From there, the blood is transported all over your body by the various arteries, to your head, arms, abdomen, legs, and so on.

As the blood flows through your veins, your organs and cells draw nutrients and oxygen from the blood, which they use, while returning waste materials to the blood. The resulting low-oxygen blood flows back to the heart, where it enters the right atrium.

From the right atrium, the blood flows to the right ventricle and on into the small circulation.

Since the blood in the large circulation has to cover a larger distance, through the whole body, the pressure on the left side of your heart is greater than on the right side of the heart.