What does "functional fitness" actually mean?
The honest answer is that it has no agreed definition, and the term is used to mean several different things — some useful, some purely marketing.
The defensible meaning: training that transfers to a specific task. Function implies a function — something you are training for. Under this definition, training is functional relative to a goal: carrying shopping, getting off the floor unassisted in old age, lifting a child, or a sport-specific demand. What is functional for a firefighter differs from what is functional for a sedentary 75-year-old.
Understood this way, the concept is genuinely useful, and the clinical version — functional capacity in rehabilitation and older-adult care — has a clear meaning: the ability to perform activities of daily living.
The marketing meaning is weaker. The term is often used to imply that exercises resembling real-world movements — standing, on unstable surfaces, with odd implements — are inherently superior to conventional gym exercises. The evidence does not support that.
Where that claim breaks down:
Resemblance is not transfer. An exercise that visually mimics a task does not necessarily improve it. Transfer depends on the physical qualities developed — strength, power, range of motion — more than on the movement looking similar.
Unstable surface training reduces the force you can produce, so it develops less strength than the same exercise on stable ground. It has a place in rehabilitation and limited use for general strength.
"Non-functional" exercises are often the most useful. A heavy squat, deadlift or press builds the strength that underlies almost every physical task, without resembling any of them. Strength is the most transferable quality there is.
What actually produces broad capability: general strength, adequate aerobic fitness, sufficient range of motion, balance, and practice of the specific tasks that matter to you.
A reasonable test: ask what function is meant, and whether the exercise improves it.