All traffic to and from your account is considered data transfer (HTTP, POP, IMAP, SMTP).
Thus, each time a page of your website (images and text) is visited by a visitor, the files contained in this page are transferred from the server to the user's device (computer for example), which generates a data transfer.
In the same way when your email client, Outlook for example, downloads an email in POP or IMAP, the transferred data is counted.
The monthly bandwidth of a hosting plan is defined as the total of data transfers made from and to the account during a calendar month.
Information, such as page templates, is stored in the web browser cache of the visitor's device when the visitor views multiple pages. This caching reduces the consumption of bandwidth as well as the use of your account's resources (CPU, RAM, etc.).