What is a category?
Categories are collections of groups which all have something in common between them, and groups are in turn just collections of items which all have something in common between them.
In this sense, think of a category as a “department” in a supermarket. You walk in and immediately see different departments such as “jewellery” or “footwear”.
And inside these departments, you have aisles, which in turn hold certain items, and this is how groups and their associated items behave within your categories.