JavaScript is a high-level object-oriented multi-paradigm programming language. A programming language is a language that we use to write code which helps us to instruct our computer about the tasks it needs to perform.
JavaScript is high-level language which means that we don't need to worry about memory allocation and other complex things. Being a high-level language JavaScript is easier for humans to read and understand.
JavaScript is based around the concept of objects and they are the core building blocks of the language. JavaScript also supports multiple programming paradigms meaning that we can use different types of programming styles to write our code.
JavaScript is mostly used for web development along with HTML and CSS. These three technologies are the core fundamentals of creating great web applications.
HTML is used to structure the contents of the website, CSS is used for styling while JavaScript is used to add functionality to our web application. JavaScript can be used to create almost any type of application and not just web applications. We can use it to create mobile apps, desktop apps as well as run it on our back-end servers.
There isn't anything you can't build with JavaScript.