Asset Library is used so that you can use the already saved chapter items which can be either pdf, text, video or quiz.
In the admin dashboard, select the "Course Builder" option for the course you want to add the chapter items.
After clicking on the "Add Chapter Item" a side bar opens with different options to add the chapter items, click on the "Asset Library" option.
You'll see an option to choose the items from the Asset Library, click on it to view the list of items already saved items.
Select the item you want to add to your chapter and Submit your requirement.
The item is added and gets visible below the Chapter, click on it and you can make changes there as well.