Bandwidth is used as a synonymous to refer to :
• DATA TRANSFER: is the amount of data actually transferred, between a computer and the internet. It is often measured in MB or GB.
A bandwidth limit exceed related to data transfer, can be compare to prepaid cellphone minutes, when you run out of minutes your cell phone would stop working, the same with your site, when you reach your data transfer "monthly" limit or the total amount of data that your visitors can view/download from the site, the access to your site wont work.
• SPEED TRANSFER: or bandwidth is the amount of data that can be transferred in a fixed amount of time, often measured in Mbps.
A Hosting Services Provider sells access plans based on speed, limiting bandwidth according to each plan. Plans with higher bandwidth limits are more expensive, but faster.
A bandwidth limit exceed related to speed transfer can be compare to a highway and its lanes. Each plan has a fixed amount of lanes assigned. If you have more cars or visitors than lanes then you have exceeded your speed or bandwidth capacity
A Bandwidth Limit Exceeded normally occurs when you have a low bandwidth limit (data or speed transfer) and a popular site. Solution: Increase your bandwidth.