Element pool is where you keep inactive form elements. By default you have one pool name Uncategorized.
When you delete a page, or a container type element, all the children are moved inside this pool. This default pool can only be renamed and cannot be deleted.
You can how-ever add more pools. The process is same as adding pages to the form builder.
Right now, the functionality of pool is to hold inactive elements. In future we will add question bank functionalities around it.