The choq of the "red heifer" (parah 'adummah in Hebrew) is given in the nineteenth chapter of the Book of Numbers (which will be read in synagogues throughout the world on the morning of the upcoming 3/29). It is the most mysterious mitzvah in the entire Torah. In fact, though Solomon's wisdom enabled him to adduce the reasons for all the other mitzvot, this one utterly escaped him. Only Moses attained understanding out of pure revelation from G-d.
There are two types of mitzvot: mishpatim ("judgments"), which can be explained rationally, and chuqqot ("statutes") which rest on no human understanding but can be accepted only as the word of G-d. The "red heifer" is the most famous of these latter.
In reality, however, at some point beyond mere human understanding, the two blend into one. For even those commandments which human reason cannot justify is nevertheless purposeful and justified in the Mind of G-d, and the most rational of commandments (such as "thou shalt not kill") in fact rest ultimately on the authority of our A-mighty Creator and not on its apparent rationality to our frail minds.
G-d's commandments bind us because they are from Him, whether or not we understand the understanding.