Find a picture or diagram of a heart on the web that explains the anatomy of the heart. You'll see that there is a wall in the middle that divides the left side of the heart from the right side. Blood returns unoxygenated from the body into the right atrium
and then goes into the right ventricle
where it is pumped to the lungs for reoxygenation. When it returns to the heart it is reoxygenated and enters the left atrium and then the left ventricle. The left ventricle then pumps the blood carrying the oxygen back out to the body and the cycle begins again.
So at any given time the blood on the left side is oxygenated while the blood on the right side of the heart is unoxygenated. If you have a hole in the wall (or septum) the unoxygenated blood mixes with the oxygenated blood (it leaks through the hole). When the left ventricle pumps the blood out to the body it has less oxygen in it than it is supposed to have. The severity of the symptoms you have depends on how large the hole is.
This hole is present in all fetuses before birth. It usually closes right before birth. Sometimes it fails to do so or doesn't close all the way. It's usually noticed at birth because babies will be cyanotic, hence the term "blue baby". It usually has to be corrected surgically.
I hope this helps. Follow up with your doctor and find out if yours is severe enough to require further treatment. God bless.