It means that during the 14 days your ticket is good (expiring) or forever (non-expiring) that you can make 6 visits to either Disney Quest, Blizzard Beach, Typhoon Lagoon, or Pleasure Island. Each visit is good for one day. So you could go to BB in the morning, go shopping in the afternoon, and go back to BB in the late afternoon and that would only count as one visit.
You could actually get 6 full days use out of these, at least at a place like DQ or the water park. However, if you went to say BB in the morning, then PI in the evening, this would use 2 of your "credits", even though it was the same day. Also if you used one of your theme park days on the same day you went to say BB in the afternoon, it would still use a credit.
Basically the credits are entirely different from the theme park admission, except they have to be used during the same time period if it's an expiring pass. Each one is good for an entire day (or hours of operation) at that park and you can use more than one per day. Like any ticket, they can't be shared (ie you couldn't add them to your ticket only and have DH use 3 and you 3). In general if you're doing 2 or so of these, it's cheaper to just add it on and then you can do up to 6 - go to PI each evening for a couple hours, hit a waterpark for a couple hours on a hot afternoon, etc. Really - it's a good deal!
