A function is an operation you perform to a number or expression. For example, my function could be "add five to the input". Now the input is what you have before you use the function; it's what you put into the function. For example, my input could be the number 7. If I put it through my function of adding five, I get 7+5=12. 12 is the output; what you have after you put it through the function. Hope this helped.