Once you complete a lesson for an item, it will be considered a newly learned item. New items are items that you have done a lesson for, but may not be truly familiar with yet.
A new item becomes a familiar item once it has been reviewed multiple times in a row successfully.
You can see your current set of new items by looking at the new items section within the Course progress widget of the dashboard.
Some items have other items they are dependent on. These are referred to as dependent items.
For characters specifically, all dependent sound and component items must become learned (i.e. new items) before the related character items show up in lessons. You can see more details in item lesson order.