"It’s true healthcare. Functional Medicine is a distinct medical model—separate and, as you’ll come to learn, vastly different from the conventional approach—one that promotes wellness and helps both prevent and treat chronic disease, the biggest health problem we face today. Although its concepts are not new, in this way, its approach is revolutionary. Functional Medicine is the future of medicine."
by Chris Kresser

He goes on to provide a great description in his blog post in July this year & well worth a read for a clear explanation.

Dr Rangan Chatterjee exposed the masses to the functional medicine approach to solving complex chronic medical conditions. Although the terms functional medicine were never used Dr Chatterjee has undergone training with the Institute of Functional Medicine and applied those principles throughout the TV series.