Categories are tiered, as you would expect. When creating a category, you can select its parent, and it will then appear as a subcategory of the parent.
This is where you input a default description for your products.
Now this is spun text, which means you will have various paragraphs in your description, with 2,3 or however many variations of that paragraph so that you're varying your content.
The structure is like this:
[["First paragraph, version 1", "First paragraph, version 2", "First paragraph, version 3"], ["Second paragraph, version 1", "Second paragraph, version 2", "Second paragraph, version 3"], ]
and so on. You can of course just have one version of the text, but you have to keep [[" at the beginning, and "]] at the end.
This is the default image for products within that category, which will show unless the product is given its own image. When creating the product, you can select whether or not to use the category's default product image.