Altogether we've got for chambers. These are the bellies of your heart. In olden days, when guests came into a house, the first room they visited was the atrium. When the blood comes back to the heart, it comes to the atrium. Now, to make the circulation work, your heart has valves, sort of like gates between the top and bottom chambers.
Valves keep the blood flowing in one direction.
Without valves, the pump won't work. It's a cycle. Every time your heart pumps, blood leaves your left ventricle and goes up to your system. Then it comes back to your right atrium and goes down to your right ventricle, then it's back up to your lungs and to your left atrium and then down again to your left ventricle. It happens all day, all the time.