A Satellite broadcasts a few watts of microwave signals from the geostationary orbit 36,000 kilometers above the earth. The transmissions are also broadcast over a wide "foodprint" area. The satellite signals suffer an attenuation of approximately 200dB, while making this 36,000 kilometers journey from the satellite to reception points on the ground. The satellite signals which finally arrive are extremely weak.
A dish antenna is used to collect these weak satellite signals over a large area - the surface of the dish. These signals are then focused by the parabolic curvature of the dish, to the focal point. Even at the focal point, the satellite signals are only a few micro-watts.