Hello and welcome to the forum. I encourage you to speak to your own doctor if you have gotten a report that you do not understand the terminology of and what the results mean in regards to your health. They are the best source of your information
However, we will try to help. The mediastinum is the thoracic cavity between your lungs and we all have lymph nodes there. So, basically there are lymph nodes in the middle of our chest and we all have those. The trachea, esophagus , aorta , pulmonary veins, and the central portion of the lungs are all in the same area. Basically these drain the lymph fluid from the lungs. The term sub centimeter obviously refers to size meaning less than a centimeter.
Basically, this is likely to be normal. But please speak to your doctor if you are concerned. Let us know what they say.