Numbers on certain letters

I notice that there are numbers on certain keyboard letters, i.e. J has a 1, K has a 2 and so on.
What is this for? I imagine its some sort of function key

thanks

 

When the "Num Lock" function is turned on, the letters on those keys are replaced by the numbers, to approximate a numeric keypad like on full-sized keyboards.
Laptops have been doing this for a long time since there isn't enough room for a numeric keypad. People who need real numeric keypads (for example, people who are used to operating 10-key machines) will either get an external numeric keypad, which connects with USB, or an external keyboard with a built-in keypad. But, this is good for occasional use, like balancing your checkbook. I almost never use mine; I just used the number keys on the top.