MooTools is designed in such a way that instead of modifying the MooTools Core files, you use its API (Application Programming Interface) to utilize MooTools function. This makes MooTools easier to maintain your projects and makes it easily extensible.
A plugin is a JavaScript file that's dependent on MooTools but is not part of the standard MooTools code base. We'll learn about writing plugins in a later chapter, but for now, just think of plugins as MooTools extensions that do a specific function, such as hiding and showing a page element or managing browser cookies.
Note
It is important to note that—unless you know exactly what you're doing—you shouldn't modify the MooTools Core files. Even if you know what you're doing, it's best to keep the file "as is" so that it's standardized and can be easily updated.