The Inventory page is where you can add, delete, and edit your rentals and accessories. This is where you'll be able to add item descriptions, dimensions, pricing, etc. You can create different types of inventory to serve specific needs like creating an accessory and making required to a rental, or creating a vendor item so you can generate a purchase order.
Here is an article on creating items.
Here is an article on accessories
Here is an article on pricing
and here is an article on deleting items and making them inactive.