Java is a programming language.
The full description is in the link, but in short we can conclude that a programming language is just a way to communicate with the computer.
When you have a schoolproject you probably create a folder on your computer and maybe a sub-folder, in which you will save the documents of your schoolproject. This exactly how codefiles are structured in Java.
Why you need this structure will be discussed later on!
We will call this: the structure OUT-side of the File or the structure OUT-side of the Class.