The 'Group by' selector is used across Seelk Studio as a means of seeing collections of product offers organised according to a specified grouping.
Once you've added product offers to Seelk Studio and their tracking has begun - the default 'groups' you'll have are:
Product offers are grouped according to the marketplace in which they exist
Each product offer in your catalog exists within one of Amazon's categories. For example, toy balls for kids exist within the category 'Toys'.
Your product offers are given a 'brand name' by Amazon.
Using 'Attributes' as groupings
You can create 'Attributes' and link product offers to attribute values in order to make customized groupings.
You are selling different towel models and each type of towel is sold in five different colors: blue, red, yellow, green and pink. You could easily create a 'color' attribute that would group these products according to their colors.
You would make a 'color' attribute and create 5 sub-values (one for each color).
Thereafter you can link the individual product offers to their relevant values.
To learn more on custom attributes, please read the article: