Prorate means that you will only pay for the ratio of the days that you have used for that particular month. 
Both monthly fees and data will be prorated for the first month. 

※ The first month's bill of your contract is prorated, which means that you will only pay the amount starting from the start date of your contract to the end of the first month.

※ As you are only paying for the days that you use the product for the first month, the data is also prorated. This means that the allocated data for the first month will also be the amount from the start date of your contract to the end of the first month.