According to the Mayo Clinic, "Functional fitness exercises train your muscles to work together and prepare them for daily tasks by simulating common movements you might do at home, at work or in sports. While using various muscles in the upper and lower body at the same time, functional fitness exercises also emphasize core stability."
The point of the article is that you should be doing multi-muscle exercises instead of targeting just one. For example the author states one should do dumbbell rows instead of tricep kickbacks. The dumbbell row works your shoulders, upper back, and biceps. I do know that with my kettlebell training I am incorporating multiple muscle groups. So I guess I have been on target of what I should be doing without actually knowing it. Now I do still do some single muscle targeted exercise. I just cannot give up the tricep kickbacks!!