"Rebill" means that at the end of the period you have paid for, you will be automatically charged again for the next period.
This is so you can enjoy continued access, without having to re-subscribe every month. If you don't like this idea, most of our clients (the website you are subscribing to) have a few other options - look for "no-rebill" subscriptions.
An example of rebilling is:
You join on March 17th, for $12, for 31 days access.
On April 17th, you are rebilled another $12.
On May 17th, another $12, and so on.
At any stage, if you want to cancel these rebills, you can. You'll have access up until the next rebill date, but you won't be rebilled - your account will be closed on the last day.