This is an extra field you will need to edit after you add the Google Shopping app. The values are according to Google's product taxonomy: https://www.google.com/basepages/producttype/taxonomy-with-ids.en-US.txt
For your own products you need to find the categories that are relevant. Shopify uses the same list for its autocomplete.
Here is an example:
You'll see that the Google Product ategory name is in Dutch, its different in every language :-)