(Note: It will be helpful to review normal heart function before reading this.)

When there's a large defect between the atria, a large amount of oxygen-rich (red) blood from the heart's left side leaks back to the right side. Then it's pumped back to the lungs, even though it's already been refreshed with oxygen. This is inefficient, since blood that's already been to the lungs is returning there, and blood that needs to go to the lungs is being displaced.

Many children with this defect have few, if any, symptoms. Closing the atrial defect by open-heart surgery in childhood can prevent serious problems later in life. Not only that, the long-term outlook is excellent.

Copied from the American Heart Association Site