Recent chest x-ray says mediastinal fat or pericardial cyst lower right hemithorax. they compared to chest x-ray from 2015 and it was present then. cardiologist said x-ray was normal as did my pcp. i read this info on my chart. what does it mean?

Changes seen are chronic, somewhat poorly understood, but not believed to be causing you any symptoms.

Considering that this was the opinion of three different individuals, I would suspect that this is of no significance. CT and MRI might be useful. If the "abnormality" was present in 2015, likely that it is a benign lesion. Given the question, it might be reasonable to get additional non-invasive studies.