what does appendchild do in javascript

function createMenuItem(name) {
    let li = document.createElement('li');
    li.textContent = name;
    return li;
}
// get the ul#menu
const menu = document.querySelector('#menu');
// add menu item
menu.appendChild(createMenuItem('Home'));
menu.appendChild(createMenuItem('Services'));
menu.appendChild(createMenuItem('About Us'));const parent = document.createElement('div');
const child = document.createElement('p');
const appendValue = parent.append(child);
console.log(appendValue) // undefined
const appendChildValue = parent.appendChild(child);
console.log(appendChildValue) // <p><p>
var element = document.getElementById("div1");
element.insertBefore(para, element.firstChild);