It means you will be notified for events such as wall posts, status updates, new friends etc. about that person. You can turn certain functions off for instance if you have no interest in knowing who...
if you subscribe, they will ask for your bank details and take a subscription fee from your bank every month.
therefore, you are esentially paying to be a part of the web site.!!!!
Although it does very from Provider to Provider, what they are usually saying in this instance is that if you already pay for the On Demand service, you won't pay anything extra for that content....