You will be charged immediately, and then again when your subscription auto-renews on the last day of every month.
3 Month Prepay
You will be charged immediately for 3-months. Then, you will be automatically renewed at the end of your term (on the last day of that month) for another 3-months.
6 Month Prepay
You will be charged immediately for 6-months. Then, you will be automatically renewed at the end of your term (on the last day of that month) for another 6-months.