Can you help me understand the concept?

There are two ways to write in a class. The first is to write methods, getters and setters, and so forth. Those are actually non-static methods that are only used when a class has been instantiated first. That is, a new object has been created. In the case of the ComicBookRepository, you'd have ComicBookRepository comicBooks = new ComicBookRepository(). And then you could use your methods, getters, setters, and so forth. These (non-static) functions are called from the class.

However, this leaves us with a bit of a disadvantage, as in the case of the ComicBookRepo. We don't want to have to create a new object every time in order to use the Repo because that would be an expensive operation to receive back a single array. So, instead, we use static keyword to allow us to call the method on the class itself. In other words, we're allowed to use it without creating a new object, because the method is not attached to any one single instance. Where, as in, ComicBookRepository comicBookRepo = new ComicBookRepo(name = "John Smith" ), the name John Smith is associated only with comicBookRepo, the getRepository() is not; it's attached to ComicBookRepository and can only be called on ComicBookRepository; not on comicBook Repo, for example.

This is definitely a tough concept at first, but it's also a very powerful feature of most OOP programming languages, and one that you'll definitely want to have down pat, because of what it allows you to do to simplify your code.

Hope that helps.

The advantage of making the the array static is that there is only one instance of the array. So, every time we want to access the _comicBooks array, we don't have to create a new instance of ComicBookRepository. This is an application of the "DRY" principle. This avoids copying the object for no reason, when we only need one.