I don't know the exact details but basically the site has a set bandwidth. Bandwith is the amount of traffic the site can handle in a given time.
The easiest way i can explain is we get a set amout of bandwidth similar to a full tank of fuel. Every time the site is used it uses a bit of bandwith/fuel. We have used to much bandwidth so its like running on an empty fuel tank.
Hope that makes sence? but it is quite late