Since there's 4 heart chambers in human heart, there is the left atrium and the right atrium.
The common function of atria is to receive blood transported from the veins. Which will then be pumped into arteries after leaving the atrium and the ventricle, which is the lower chamber of heart. The right atrium receives blood from the vena cava, which is transported from all the body parts, except lungs, while the left atrium receives blood from the pulmonary veins, which transport blood from the lungs.
Below the atria, there's also valves. When they're open, they allow blood to leave the atria, and towards the ventricle. They tend to open when the blood has too much weight which will push them open.
Usually, atria is smaller than ventricle, as they're not the location for the muscles to pump the blood away from the heart.
