Google play store reviews: For an older version
It simply means that the comments posted by reviewers pertain to older version of the app , immaterial whether they are good or bad

Other comments which don't have this comment are for the current version, so that's what you need to pay attention to (Bugs in older versions may have been fixed as is usually the case)

As clarified in this SO question, there can be only rating for app by a user , irrespective of the versions. Hence the need to differentiate old reviews from current version reviews

Seeing wbogac z's comment, found this blog which mentions that their is a separate versioning that is not visible to the users and older reviews are likely based on that. 

Edit: See this for versioning details from a developer point of view 
App updation- Google Play Store wants to update an older version of app when latest version is already installed?

Requesting developers to confirm or correct