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 $28.26 /mo but you received your Welcome Pack in the middle of a 31 days month, let's say on the 15th, you only have half a month to use your service: the pro-rated charge will be $15.50!
Pro-rated charge = {(Total days in month - Number of days base plan is used) / Total number of days in a month} * Base Plan subscription price
Hence, in this case,
$15.50 = {(31-15+1) / 31} * 28.26
*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 $28.26 /mo Base Plan.