In the old interface, you would go to Products ->Categories in your left hand menu to view your existing categories.
Furthermore, if a specific category had its own subcategories, you would see a number on the category that would let you view its sub-categories.
Now, all categories can be viewed from one place at the same time. You’ll simply go to Products ->Categories and you will see your store’s main categories.
And, if a category has subs, you will see an > icon on the Parent Category that can be clicked to view the sub(s).