JavaScript
JavaScript is the language that makes a page do things when a visitor clicks, types or scrolls.
In a simple example
When you press Add to basket and the number on the basket changes at once, JavaScript did that.
Why it matters to you
Anything that moves or reacts on a page usually needs it, and it also needs care because it runs on your visitor's device.
Do not confuse with
JavaScript is not Java. They are two different languages with similar names.