HD: The resolution of a video, or video quality.
The joke is AD(HD), which initially stands for Attention Deficit Hyperactivity Disorder, which I have. However, the pun is essentially saying Attention Deficit in HD. The second part (AD4K) means Attention Deficit in 4K. HD quality is poorer than 4K quality. Sorry I didn't answer the initial question, but if I were to guess, your doctor is talking about something that makes up who you are in real life.