In Accelerated Reader Bookguide, the Active List is the book list that the program considers to be "open". You can see this list when you select Active List while viewing your book lists.
In the table of book lists, the active list is shaded. See the "Dogs" list in the example above.
When you search for books, the active list is the one that is selected by default, and therefore it is the list that you can add titles to. However, you can select a different book list from the drop-down list.