Fluid on the lungs is also known as Pulmonary Edema which results when the alveoli fills up with excess fluid that seeps out of the blood vessels in your lungs instead of the air like it's supposed too.
When this excess fluid fills your lungs it causes problems with the Carbon Dioxide and Oxygen that is exchanged and this can cause difficulty breathing.
Sometimes the fluid on the lungs may go away on it's own but in some cases it requires a surgical procedure to remove the fluid off your lungs.
If you don't get the excess fluid removed from your lungs you could die so don't hesitate to see your doctor if you do get fluid on your lungs.