I went to ER with an episode of lightheadedness. Transesophageal echo showed medium to large pfo. What does this mean?

What is a PFO. A PFO is a small hole between the atria (small chambers) of the heart. We have these during development / as infants, the holes normally close as we age. Medium to large PFO can sometimes cause lightheadedness, sometimes it causes no symptoms. When it affects cardiac output, or exercise tolerance, it may be important to address. Your cardiologist will probably tell you a lot more soon.
Not much. 10% of the population has this abnormality. You were probably born with it. Most likely it is not the cause of your lightheadedness. Further studies may be warranted however.
Hole in heart. There is a connection between the left and right sides of your heart. If a clot from the leg for instance crosses that hole and goes to your brain it could produce a stroke. So sometimes it is decided to close the hole or to put the patient on blood thinners. Since such holes are very common, your symptoms may well be unrelated to that pfo, patent foramen ovale (the technical name for the hole).