An LNB, or Low Noise Block downconverter, is a the small box on an arm that points toward the center of a satellite dish. It receives the signal and sends it to the satellite receiver box that connects to your television.
LNB Concept
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The cylindrical shape of a satellite dish allows for tiny margins of error in alignment, which can make a huge difference given the distance between the dish and the satellite. The LNB is positioned such that wherever the signal strikes the dish, it is reflected into the LNB's path. This greatly reduces the level of precision needed in aligning the dish, though it must still be pointing in the correct direction.
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LNB Terminology
The "low noise" in LNB refers to how the LNC amplifies the signal received by the dish, creating more signal and less background noise. "Block downconverter" is because the LNB works with a block of frequencies and reduces each from a frequency suitable for transmission from a satellite to one which can be handled by the satellite receiver.
Multiple LNB Outputs
Although a dish will normally only have one LNB, some LNBs are made with two or more output sockets, each of which can connect to a different satellite receiver box. Having multiple outputs is the only way for the different receivers to be tuned in to different channels at the same time. This type of dish is often marketed with terms such as "dual-LNB" or "quad-LNB."