Your heart has a left and a right side and each side has an upper and a lower chamber (an atrium and a ventricle). There are 4 valves in the heart, 1 between each atrium and ventricle and 1 at the exit of each ventricle. The valves make sure that blood flows in 1 direction through the heart like a one-way traffic system. They open and close at particular times during the normal beat of the heart. They open to let the blood through at the right time and then close to stop the blood flowing backwards.
The image below shows the 4 chambers of the heart and the location of the 4 valves.
Image credit: Bruce Blausen Wikimedia Commons
Video: British Heart Foundation - How does a healthy heart work?
This video shows how the blood flows through the heart. It may take a few moments to load.
(British Heart Foundation, UK, 2014)