Here you can create pricing to attach to your rentals. Generally when you create a new inventory item, you give it a price as you are creating it. That action creates a unique Price Category and places the new inventory item in that unique price category automatically.
However, you can create a price category here that is used for more than one rental item. For example, if you have several "medium sized bounce houses" that you rent out (all the same price), you can create one price category and place several items into that price category. In this scenario, if you change the price for one of those rentals--all of the prices will change. In the future, if you want to change the prices of the individual rentals, you will need to create a new price for each rental items.