Bandwidth, also known as traffic is the measurement of how many people have accessed your site over a period of time. When you view a site your computer is exchanging information with the server that the site is on, if too many people access the same information all at the same time then the server will start to run slow or in the worst case scenario crash.
For this reason monthly limits are put into place and if you go beyond this limit of your hosting package, your site will be inaccessible until either you buy more bandwidth or until the start of the following month.
Most companies, including Wight Hosting, advertise 'Unlimited Bandwidth' packages. For this to be true the server that your site is on would have to have unlimited resources such as memory, hard drive space and processing power. Obviously no machine exists, else we would all have computers that never needed upgrading.
Well I can't comment on all hosting companies, but in the case of Wight Hosting. It means this:
"UNLIMITED" USAGE POLICIES AND DEFINITIONS
The term “UNLIMITED” is used to describe the amount of resources in the web hosting plans provided by Wight Hosting, which is not set to a predefined limit value (for example - unlimited bandwidth, unlimited disk space, unlimited emails or other unlimited resources). By not setting limits on key resources, we are able to offer our customers a simple, consistent pricing scheme as their websites demand more and more resources. As a result, a typical website may experience periods of great popularity and a resulting increased storage need without experiencing any associated increase in hosting charges.
In all cases, when a service is listed as “Unlimited“, it is intended for normal web hosting usage only - all activities, which may result in excessive resource usage inconsistent with regular usage patterns, are prohibited.
Our Unlimited service offers are not designed to allow a single user to influence the overall server performance. Wight Hosting will make every effort to provide its customers with additional resources whenever possible, including the relocation of a customer's hosting account to a newer, faster server. However, if the resource usage reaches a level where it may influence the overall server performance in a negative way, Wight Hosting reserves the right to take corrective actions. Such actions may include disconnection or discontinuance of any and of all offered services, removal or deletion of customers' websites, website content, electronic mail, or termination of a customer's account according to Wight Hosting's sole and absolute discretion.
The above is taken from the terms and conditions that can be found at the bottom of the order form.