One fish, Two fish,
Stuck fish, Sad fish.
Bandwidth is the communication capacity of an electronic pathway, such as a communication pipeline. Data flows quickly and smoothly when the amount of traffic traveling in the pipe is small relative to the size of the pipe. When the amount of traffic exceeds the capacity of the pipe, the speed at which data travels begins to drop.
Imagine the network wires as pipes - the larger the pipes, the more data can pass through.
While using the Internet, you are using bandwidth to transfer data. If you are abusing bandwidth, you are taking up a larger part of the pipe which normally would be divided more equally among everyone else.