dish is a large structure
shaped like an upside-down wok
with an aerial
on top of it. The purpose of a satellite dish is to receive (and at times send) signal
s from a satellite.
The reason why you need a dish is to concentrate the signals from the satelite onto one point where the receiver is. As you get further from the satellite signals from the satellite disperse making the signal weaker. The dish is shaped in such a way that the radio waves that have spread out are reflected to a focal point where the reciever is thus increasing the strength of the signal.
Satellite dishes come in all sizes and are used for different purposes. Many ISPs use them to provide a broadband service, people use them to get satellite TV, NSA uses them to communicate with their satellites and any missions that they may have.