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 :-)
I sell books in my store and I've identified the following google product categories:
784 - Media > Books
543542 - Media > Books > E-books
543543 - Media > Books > Print Books
Trouble is, some of the titles in my catalog have variants (hard cover, paperback, epub). When I try to manage my products Google Channel only sees the top line and won't let me assign a category to each variant. Should I just assign 784 - Media > Books to the top title?