For instance, instead of writing the verbose and cumbersome:
var p = document.createElement("p"); p.appendChild(document.createTextNode("more content"); document.getElementById("container").appendChild(p);
In JQuery, you can write:
$("#container").append("<p>more content</p>”); // the JQuery API can be accessed from the “JQuery” object or the shortcut “$”.
Historically, JQuery was important during the browser wars because browser vendors implemented the DOM API standard inconsistently, and JQuery was the best way to ensure consistent behavior across them. That is less relevant nowadays as browsers gradually converge.
Here is the official JQuery API documentation
You can get started at learn.JQuery.com.