What is reverse blood flow in heart chambers?

It is a sign of a disease like left or right sided heart failure or tetralogy of Fallot where the unoxygenated and oxygenated blood gets mixed up.

This happens because the heart's valve could no longer regulate the blood flowing through it. In left sided heart failure, the oxygenated blood goes back to the lungs instead of going to the body so those who have it will have respiratory diseases. Meanwhile, in right sided heart failure, the unoxygenated blood goes back to the body instead of going to the lungs which is why those who have it will develop edema on their extremities.