jQuery main function is the ‘$(document).ready() function’. Although it’s many way to start jQuery, Always use this function to start using jQuery is good. example:

$(document).ready(function() {
$('.header-company').addClass('highlight');
});

This will add class named ‘highlight’ to element that have ‘header-company’ class. This is used generally for changing / adding class for CSS. So, when the document was loaded, the jQuery command will add the CSS Style.