An LNB is the box of electronics on the end of the dish boom arm. It acts like a aerial masthead amplifier. The signal (microwaves) reflect off the dish and into the feedhorn to the LNB. Here it is amplified and the frequency is lowered. The lower the noise figure the better.
i.e. in simple terms, it pulls in the signal better. The signal is then sent down the coaxial cable to the satellite receiver.