Every reseller package is allocated a certain amount of data transfer for distribution between hosting accounts. This is the amount of data your sites/customers are allowed to transfer collectively. You may portion this out to your sites as you see fit, and you may keep as much in reserve as you'd like too.
It is rare that an account uses its entire data transfer quota in a given month, but should this happen, the cost for additional data transfer is only $0.80/gb/£0.40/gb, and you'll receive an email prior to your account reaching its limits.