Everytime someone looks at a file that loads from your diskspace (the part of Dreamhost that is "yours"), some of your bandwidth is used up.
Think of it this way: if you have a video that is, say, 7MB big, 7MB are used everytime someone watches the video. If the video is viewed 10 times, 70MB will be used; if it's viewed 100 times, 700MB will be used.
The same is true of any web pages you post. You can add up the total of your html code and all images to get your size. A page with 15k of code and three images that are each 50k totals ~165k everytime someone loads that page in their browser. You can do the math from there.
You may hear people on this board and elsewhere discuss "hotlinking" which is where someone posts a file that links to space that isn't owned by them. If somone streamed your music video by linking directly to your file, your bandwidth would be used up (stolen) by someone else. You can help prevent this by enabling increased security in your web panel.