This level focuses on words ending with able or ible. A general rule of thumb is that able works as a suffix, so you can generally remove the able ending and still have a root word. The ible ending is generally used when that is not the case. So a book is readable (because read is a stand-alone root word), but something that can be seen is visible (because vis is not a stand-alone root word).
There are exceptions to that rule, however, such as accessible or flexible.
One additional note: for root words ending in a silent e, that silent e drops before the suffix is added. So the correct spellings are lovable and disposable, not loveable and disposeable.