My x-ray report says: mediastinum: mildly enlarged cardiac silhouette. what does this mean. is it serious?

This means that your heart looks bigger than normal on your chest x-ray. You need further testing to determine what is causing this. You should see a cardiologist for an EKG, if not done already, an echocardiogram and perhaps a stress test. The testing needed depends on your symptoms and a cardiologist will be able to sort out what testing you need to make the diagnosis. Best of luck to you!

I agree with Dr.F. I would add that technique of performing the chest x-ray is important in that an AP view is notoriously inaccurate in assessing heart size(exaggerates it) compared to a PA view as is often is taken in the emergency room setting or at the bedside.