What are berry grains?
The term rouhe in Finnish means "something coarsely ground" [1], [2]. (Google translates it as "groats", but that's a bit too simple.)

Double-checking with the English, German and Italian version of the shop, they always use a version of "grain or granule" in the specific language:  Note that "granello" in Italian, for example, which is about the shape, not a cereal.

As the product sites for the various "rouhe" claim that they use the whole berry, it means whole dried berries (aronia in your case), coarsely ground. The weight and volume data given is "250ml / 85g". I confirmed "dried" (as opposed to compote-style fruit preparations) by weighing a cup of fruit tea: 76 g, but of course a much coarser texture.



Edit:

Another website writes (according to google translate):


  Berry powders and grits, prepared by grinding whole dried berries, without adding or removing anything.




Edit 2:

The manufacturer suggests adding the granules into dairy products or porridge or using them in baked goods.