Object categories are among the most useful features on the platorm - they allow you to categorise and keep your content in order.
When you go to Configuration/Everything else/Object categories, you can see the list of categories that were already created. You can use the "Add category" button to create a new one.
When you add a category, just have to name it, categories don't have any other properties.
If you check the category and choose "Delete", you can choose replacement category for elements that are in the category you want to delete.