The price category is used for having multiple price slabs for a single product.
A product will have a Purchase Price by default. It will also have a Sales Price.
But apart from that it could also have a Dealer Price, Special Customer Price, Festival Season Price, etc.
You could create a template for the price category using the Price Category page.
It could be accessed via Product >> Price Category menu.
You could learn more about it from¬ How to add Price Category?