Each time a visitor to your site views a page or image they transfer data between your site and their computer. The amount of data transferred through the network connection is referred to as bandwidth and so visitors are using your site's bandwidth as they browse.
Bandwidth is not the same as disk space
Storage or disk space is the space available for you to store your files, it decreases each time a new file is added to the website. The storage space available does not change over time or renew, it only changes through adding or removing files.
Bandwidth is allocated as an amount per month and the usage counters reset at the start of each month. It is calculated by counting the size of each file or web page viewed by each visitor.
For example if your web page including all the images is 2MB and 10 visitors view that web page then it is 2MB x 10 = 20MB of bandwidth used.
You can easily see how much bandwidth you have used within a given month by login in to cPanel and looking down the right column until you see the statistics block. The statistics block shows the usage of the various resources allocated to your hosting account including the current bandwidth allocation and usage.
Preserving your bandwidth
One way to prevent exceeding your bandwidth, especially if your site has a lot of visitors is to make sure that you keep file sizes small and only load images and media into a web page as needed. A side effect of this is that it can also reduce the loading time of your site.
There are may file types from simple text files through to full movies, movies are the quickest way to consume bandwidth as they are generally large.