Question

In hilly areas television reception may be poor. To correct this problem the signal is often boosted by a local relay station before being re-transmitted. Sometimes this introduces further difficulties because of the interference between the incident and the re-transmitted signal. This can be avoided if the main transmitter emits waves that are vertically polarised.

Can someone explain to me why this would solve the problems of interference? Really very interested to know :) A detailed explanation would be good