It means that your Base Plan will be adjusted and charged from the time you receive your Welcome Pack!* For example, if your Base Plan subscription is $28 /mo but you received your Welcome Pack in the middle of the month, you only have half a month to use your service: the pro-rated charge will be $14!
*Please note that your first month's bill also includes your Pay-As-You-Go expenses for the month, your Plus options (not prorated) and your second month's Base Plan charged in advance. For more details about your first bill, head here.
*Pay-As-You-Go expenses and Plus options are only available if you're on the $28 /mo Base Plan.