In other words Your Visitors. So anyone who goes to your site.
My Definition: "The amount of data that visitors to your site download."
So if one of your pages was 1kB in total then each time someone viewed that page they would use 1kB of your bandwidth.
We give you a set amount of bandwidth and this resets every month. It's similar to cell phone minutes, every time someone calls you, you lose some minutes. Every time someone goes to your site you lose some bandwidth. At the end of the month it resets back to the full amount.
We rarely ever have anyone use more then their allowed so I would not worry about it.