Every hosting account is allocated a certain amount of data transfer (some hosts mistakenly call this bandwidth). This is the amount of data your account is allowed to transfer to and from your visitors. The amount encompasses data being downloaded from your site, uploaded by you or your visitors and transferred during email downloading.
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.