It means if you log in as a current subscriber, 12 issues will be added to your current
subscription- your current subscription will be extended by 12 issues.. So, if you have 3
more issues of your current subscription to run, your new subscription will start after
that, and you get no overlaps.
If you don't log on, or are a new subscriber, you can
start your subscription anytime, and if you did this while already a subscriber you could
end up with 2 issues of the same month's magazine.