Bandwidth is an important thing you have to look in to when you are buying web hosting for your web site.
Normally bandwidth is defined as,
In that case you have to buy some bandwidth from your web host to last till to the next month. Normally 3$/GB for extra bandwidth they charge.
Here is a guide for calculating your needed bandwidth amount.
Say your average web page size is 50kb and you have visitors viewing 2,000 per day. So you will need a 3GB/month hosting minimum. So to keep it sure working if has more visitors buy a 4 to 5GB hosting.
If you are going to start a new web site go for the minimum GB limit and later you can increase it after according to the need. Most of the hosts provide more bandwidth as per the hosting MB size.