You can hover over the rating to see the info on the games store page.

A quick google shows this so it should be fairly accurate:

Overwhelmingly Positive: 95% or higher positive reviews with at least 500 total reviews.
Very Positive: 80-94% positive reviews with at least 50 total reviews.
Positive: 80-94% positive reviews with 10 to 49 total reviews.
Mostly Positive: 70-79% positive reviews with at least 10 total reviews.
Mixed: 40-69% positive reviews.
Mostly Negative: 20-39% positive reviews.
Very Negative: 0-19% positive reviews with fewer than 50 reviews.
Overwhelmingly Negative: 0-19% positive reviews

Dang it all, I forgot to include Mostly Positive because it was the one confusing me.

Gracias, companero!

By order of importance sure, Very positive or Overwhelmingly Positive could be seen as better.

I however prefer when games say Positive or Mostly Positive because you know the don't have fake or meme reviews.