42 is the Answer to the Ultimate Question of Life, the Universe, and Everything.
Currently only Giygas knows how to use the answer (he keeps the question in secret). The King of Hyrule wants to know the answer too, because he thinks that the question is "What's for Dinner?".
This was one of the main causes for the Hyrulian Civil War, though all participants are too embarrassed to admit they were all fighting over a number. It also caused several other wars, but they were all so pathetic, we are not going to mention them.