I find it's easiest to describe Functional Medicine (FM) with a definition, and then an example. FM is thinking about the entire body (physical body, but also mind and soul) when trying to identify a health issue and then restore health. For example, you've received a lab order that reveals you have a low functioning thyroid. Your medical practitioner tells you it is best to start on thyroid medication such as Levothyroxine. Yes, taking thyroid medication (which contains thyroid hormone to help increase your low levels), will likely help you feel better, but it does not fix the underlying problem.
You may be thinking - "the underlying problem was a low functioning thyroid so aren't we fixing the underlying problem with thyroid medication"? But we need to be asking, "Why is your thyroid under functioning"? Is there a nutrient deficiency? High stress? High inflammation (what is the source)? Toxic exposure? Autoimmunity? Other hormone imbalances? Gut issues? There are so many avenues to explore and figure out! Because as long as the underlying issue is going unidentified and untreated, other symptoms will remain and the thyroid medication will be necessary to keep thyroid hormone levels in range.
This is just one example of how functional medicine works. Think of what health symptoms you are experiencing - heartburn, headaches, joint pain, menstrual irregularities or ED? There are prescriptions to help with all of these symptoms, and they may be very helpful - but unfortunately they are a temporary solution. The root cause that continues untreated, will eventually contribute to another symptom cropping up and the cycle will continue.
Identify the root cause of your symptoms, correct them and enjoy restored health!