Every time someone visits a web page their browser downloads the HTML code , or other type, to the visitors own PC, along with all the graphic images etc. Let us assume that a index.html page and all its associated images comes to 50kb , 50kb of data would have passed from the server your site is hosted to the visitors PC.
Now if you have 100 people visit that page, then the data will have to be sent another 100 times, thus making a total of 100x50kb = 500kb. This is what is known as Bandwidth, the amount of data sent from the server you are hosted on to all your visitors.
In above example 5000 visitors will use up 250,000 kb or 244MB. High volume traffic sites do best to have popular pages graphics and pictures optimised as best as possible and limited to what is shown to help cut down bandwidth. Keep those large images or movies to separate pages/links to help preserve precious bandwidth
Hope this helps.