When a signal is transmitted, it has a certain effective radiated power. Radio and microwaves travel in waves and these waves tend to be dispersed by diffraction or reflection in the atmosphere over large distances. This dispersion causes the signal to scatter, weaken and be of lower effective power when received because less of the signal made it to the receiver.
What is Gain?
The difference between the strength of the received signal and the strength of the signal after something has been done to increase the strength of the signal is called signal gain or just gain. When a received signal is weak, something must be done to strengthen the signal. There are two things you can do to increase the effective strength of the signal. The first is to usea a larger antenna. The second is to amplify the signal. You can use either method, but both are usually used together in satellite communications.Using a Larger Antenna
You can use a larger antenna which will allow you to 'catch' more of the signal. With parabolic dishes, increasing the surface area of the dish increases the amount of signal received. The shape of the dish then focuses the received signal into a much smaller area at the focal point of the dish. By placing the receiver at the dish's focal point, you increase the amount of signal received. This is how parabolic dishes work.
Signal Amplification
You can also process the signal using signal boosting amplifiers. A radio signal is passed through a vacuum tube (travelling wave or Klystron tube amplifiers) or through a series of electronic circuits (solid state amplifiers) and the effective power of the signal is raised.