JavaScript is a programming language often used to make web pages respond to people. If a button opens a menu, a page checks a form, or new content appears without loading a whole new page, JavaScript may be doing that work.
You can think of a web page as a room and JavaScript as the instructions for what should happen when someone presses a switch. It can also run on servers and in other environments.
JavaScript is not the same thing as Java, despite the similar names. It is one tool among many, and it does not automatically make every page faster or better.