"Full stack" refers to the front-end of a website that deals with the user interface, the back-end that deals with data validation, as well as the website database that acts as storage from the front-end through the logic layer. Full Stack development refers to the development of the front-end, back-end, and database.
There are several "full stacks" such as JavaScript, Java, Python, C#, and more. Each of these full stacks uses a different set of tools, frameworks, and technologies. To become a Full-Stack Developer, you need to acquire full-stack development skills and learn how to use the associated front and back-end languages, tools, and frameworks so you can create your application from start to finish. With the demand for Full-Stack JavaScript developers being especially high, JavaScript would be a great place to start.