All of us have seen function keys before. They are pretty much the standard keys you would find on any laptop or computer (Except if you are one of those Macbook 2016 users). If you are a programmer, you would have already used some of them, but for normal people, their purpose is still unknown.
In various programming environments, these keys are used to compile or run a program. There are a lot of uses to the function keys that you should know. If you are using a laptop, you can access the function keys by using the function key in the left bottom corner and the function key on the top row. In most mobile devices, the function keys are also used to serve many other purposes like music controls, adjusting screen brightness etc.
Here is an image that shows exactly how useful, function keys can be: