What does document.documentElement and the if line mean?

Question

JavaScript

By clicking on the button, check if the page is scrolled to the very bottom. If so, scroll it to position 100px from the top
var button = document.getElementById('button');
button.addEventListener('click', func);
function func() {
var body = document.documentElement;
if (body.scrollHeight - body.scrollTop == body.clientHeight) {
window.scrollTo(0, 100);
}
}