Data transfer is the amount of data you can transfer between your websites and your visitors. It also covers the data transferred via your email accounts and FTP.
Pipe Ten provide you with a set allocation, should you approach this quota you will be sent an email notifying you of this, should you exceed the quota on your account you will be billed proportionally for the over usage on a per GB basis. You can also upgrade the bandwidth allocation on your account within your control panel.

A small sized web site will consume between 1GB and 5GB of data transfer per month.
You can calculate this by multiplying your average page size with the number of expected page views per month. If your average page size is 30K and you expect 50,000 page views per month, you will need 0.03MB x 50,000 = 1.5GB.