Bandwidth is actually the amount of data transferred through the web hosting account at one time. Every Web hosting account comes up with bandwidth limit on per month basis.
If your bandwidth is exhausted then all your activities related to website such as email and website will stop working and you will have to wait for the beginning of next month for getting your website & email service active again or else you will have to upgrade your current Web Hosting Plan.
Error message screenshot as below while you open your website in browser:
When you get this type of message while accessing your website; it means your bandwidth limit quota is over for current month.
Bandwidth consumption include following things on monthly basis:
Email Account via POP3
Email Account via IMAP
For more information check the bandwidth usage sample in below screenshot: