Okay, this is the best that I can explain it.
Think of a 1 litre bucket of water. This is the your total amount of bandwidth you are allowed per month by you webhost(10gig per month or whatever).
There is a hose in the bucket that allows it to drain. The hose is only so big allowing .01 litres/sec of water to flow. This is the maximum bandwidth per second your internet service provider will allow (1Mbp/sec upload or whatever)(They also usually have a maximum bandwidth per month).
Whenever someone accesses your webpage the tap on the hose in the bucket is turned on. This allows the water/data to be transferred to the client. So the client requests the webpage and your server sends the data comprised of all the images and html files(8k image, 5k for html file, 8k for stylesheet). Then the tap is turned off once they have got all the data they need for the page. your bucket is now a little bit emptier. The hose is only a certain size, so you can't get more flow/second out.
Now your bucket is a little bit emptier. At the end of the month, your host turns on the tap and refills the bucket with water. If too many people access your page and the bucket empties before the end of the month, your host will usually charge you to fill it up again with a little water/bandwidth to get you through until the end of the month. Once the new month starts, your back to your full bucket.
Simple right? Now go get a drink of water. :D