Additional Data Transfer can be be purchased to top-up your monthly allocation. The Pre Paid Data Transfer can be bought at any time and only gets used if you go over your monthly allowance, otherwise it is just carried over every month.
If you exceed your Monthly Data Transfer limit and your Pre Paid Data Transfer limit then your website will be unavailable.
All of the hosting packages are allocated a Monthly Data Transfer limit. If at anytime you feel that you might exceed this you can purchase some Pre Paid Data Transfer. For example if you have uploaded large music or movie files to your web space for users to download you might want to purchase some Pre Paid Data Transfer so the site does not go off-line. The graph at the top of the page allows you to keep track of how much of your Pre Paid Data Transfer allocation you have used.