If you take the premium, your visitors may download your files for up to the allowed amount of bandwidth without seeing the ads.

For example, if you have a 100mb file and 100Gb of bandwidth, the file can be downloaded 1000 times. After that you either have to upgrade your package.

However, I do believe that your unused quota can be levied over to the next month. So if you still have left 10Gb, the next month your allowance will be 110Gb.