What's a category?
Categories can be found on the "Items" step when building your collection. There are a few different reasons why you might want to add a category to a collection. For starters, you may want to create some visual separation between a list of items. Categories can also serve as filters, which allow a payer to easily view different categories of items.
You can add a category to a collection by clicking "Add Category" on the Items step. When creating a category, you even have the option to add a category description, which allows for subtext below the category header.
Once you've created a category, there are a few ways to add an item to a category. You can click on the three dots to the left of an item and drag and drop the item into the category box. Or, when creating a new item, you can use the "Add to category" drop down that appears to the right of the item name (note that this drop down will only appear after you've created a category).
The image below shows how categories can be used to filter items on the collection page.