.. I assume this is from a chest xray report. This is really a nonspecific statement. It could be superimposition of normal structures or represent a mass or lymphadenopathy. This could be followed with another chest xray, or examined with a ct with contrast, at your doctor's discretion.
Not much. This is just a radiologic description, really a hedge, not a disease. In other words, the radiologist who has interpreted your study (i presume a ct or cat scan) sees prominence in the area of your aorta and pulmonary artery and is just reporting what he/she sees. If there were an aneurysm, it would be stated. I wouldn't worry about it or pursue it.