Your Capitan License provides your website with a bandwidth allocation, which is much the same as a data limit on a mobile phone contract. It's the amount of data which is transferred in/out of your website each month.
If you've been notified that you exceeded capacity don't worry - your service continues as normal. If your website exceeds the allocated bandwidth for 3 consecutive months your subscription will automatically upgrade to incorporate the boosters required for the new level of data transfer.
How to check your allocated capacity and usage:
Login to your Capitan installation and to to "Settings", you will find your license usage and capacity on the right hand side of the page.