These are very basic javascript commands. You won't be able to write a calculator if you do not understand the basics.
Let outputScreen = document.getElementById("input") // This gets an element from your page with an id of "input" and stores it in a variable named outputScreen. This is self explanatory.
Function display(num){
outputScreen.value += num;
}
This is a function that you pass a value into and it adds it to whatever is in the input element.
For eval, see
JavaScript eval() Function[
^]