From how I interpret it, it means that you can use X amount of megabytes or gigabytes within the billing cycle. For example, if you went with $100/2 months for 10GB of data, you would pay $100 every 2 months, and within those 2 months you would be able to use 10 gigabytes of data. It looks like the average smartphone user uses about 3GB of data per month, but you'll have to gauge it based on what you normally use.
