JavaScript is the language of the web. It's the third part of the "Web Trifecta," controlling the behavior and interactivity of a website or web application.
JavaScript is a dynamically-typed interpreted language. That means that you don't ever have to declare a datatype (like 'int', or 'String'), and you don't ever have to compile it. Nice, right?
While it can be used server-side, JavaScript is primarily a client-side language. That means that it is executed by your browser.
It's considered to be a C-like language, as it uses curly braces, parentheses, and the same condition and loop-structures you're used to using in C/C++/Java.
This repo has a folder called samples, and each of those samples has a well-documented JavaScript file called "script.js." Reading through those should give you a good starting point for your journey into writing JavaScript!