Windows 8.1 laptop and I am not sure what is the num lock key

Hello,
I have a windows 8.1 laptop and I am not sure what is the num lock key but I see the key marked "insert" and on the bottom of that key it says "num lk" but if I press "fn" and "num lk" and I try to type the keys but they always become letters. Please help and respond to me. Thank you