jQuery is a JavaScript framework that's used by a majority of Internet sites. If you’re only interested in making minor changes it’s likely you'll never need to work with jQuery, but if you plan to make custom themes or blocks, you'll want to get familiar with it. Rock currently uses version 3.5.1. You’re welcome to use a newer version in your theme but be sure that your version is backwards compatible to 3.5.1 to ensure Rock's core jQuery plugins work correctly. You can find out more about jQuery at jquery.com.