jQuery is the most popular JavaScript library. It gives developers lots of useful functions so they can make their webpage interactive across multiple browsers.
jQuery is an open source library with a big community of contributors.
If you learn jQuery, you'll be able to use it in your own webpages, understand other developer's webpages, and have a better understanding generally about how to use libraries.
Note: jQuery functions use the DOM API (like document.getElementById). You should learn that first, if you haven't yet.