Here are some examples of things you can do with jQuery
This is not a exhaustive list (not possible)
Using methods like .html(), .text() and .val() you can get (i.e. retrieve) and set content on your pages
// get the text content stored in an element
$('p.intro').text();
// set the content to another value
$('p.intro').text('Updated text that will replace existing content');
Using methods like .prepend() and .append() you can dynamically add elements to html to your pages