can anyone tell me what these few lines of code do? The first 2 from what I understand return the first element that belongs to the class or classes in brackets ... I did not understand then the following lines ...

let menu = document.querySelector('#menu-btn');
let navbar = document.querySelector('.header .navbar');

menu.onclick = () =>{
menu.classList.toggle('fa-times');
navbar.classList.toggle('activee');
 };