Files that are loaded to or from servers uses internet bandwidth to push files along the network at various speeds. Every time you upload a file to your ISP, surf the web or use an audio application, you are using bandwidth.
Bandwidth is a bunch of wires or fibers connecting servers to a network. Depending on the grade of the wire it determines how much data is coming across the network where your web-site is hosted. When someone attempts to get more data than can be handled by the network, the whole network slows down.