Scratch

Lists are exactly what the name implies. Just a list. It has no length limit. Like a bunch of variables.

It is useful for storing variables, so that you can have a nearly infinite amount. I encourage you to experiment with it, as it is a very useful feature of Scratch. 

A list is like a book.  On every page, there is written either a word or number - some sequence of characters.  You as a programmer are allowed to add and insert pages anywhere in the book, delete ("tear out") pages, check the value of ("read") pages, and check to see whether there is a certain "word" written on any page of the book.

More generally, a list is a numbered collection of variables.  It allows you to dynamically store data (in other words, you could have a theoretically unlimited number of items in your list that are all created at runtime).  You can then access each item of the list individually.  Every member of the list functions as an independent variable: you can read and write in the same way you would to any other variable. 

I just like the book analogy.  Hope that helps clear up your question.