Here is my understanding:

Ethics is the study of what one has most reason to do or want. On that definition, it is directly relevant to instrumental rationality. And if we want to discover the facts about ethics, we should determine what sort of things those facts would be, so that we might recognize them when we've found them - this, on one view, is the role of metaethics. This post is an intro to current thought on metaethics, which should at least make more clear the scope of the problem to any who would like to pursue it.