He became a hearld of Galactus in order to spare his homeworld Zenn-La (or something like that). Now just a word about Galactus... he's older than the universe itself (he was the sole survivor from the previous universe before the Big Bang); he's so powerful that the beings that he has made his hearlds have incalcuable power; he also has to "eat" planets in order to survive ... which is why Zenn-La was in danger. Norrin Radd (the Silver Surfer) basically made a deal with Galactus to become his hearld and to search for uninhabited planets for his master to eat in order to spare his homeworld. Unfortunately, Zenn-La was destroyed anyway. In any case, the time came when Norrin found the Earth and instead of letting his master consume us, he turned on Galactus and became our protector.
