Moral relativism is the opposite of moral absolutism.
Moral absolutism is the belief that certain actions are intrinsically moral
or immoral.
For example, cannibalism. Say there is some tribe that eats their dead.
A moral absolutist would say that "cannibalism is immoral", and therefore
conclude that the tribe is immoral[1].
Whereas a moral relativist would say that, if you had grown up in a tribe
of cannibals, then eating people would be a perfectly normal thing to do,
it would be moral from the tribe's reference frame, and immoral from our
reference frame.
Yes, different moral systems can be compared along certain objective
dimensions. Objectively, cannibalism causes certain diseases such as kuru.
However, choosing which dimensions to optimise for is itself a
subjective process. Objectively, cannibalism prevents food waste :)
Relativism is precisely what allows for meaningful disagreements to occur.
Otherwise a disagreement would simply degenerate into a loud exchange of
axioms (this often happens).
My problem with moral absolutism is that it requires a healthy dose of
chronocentrism to buy into it without reference to some codified set of
morals such as a religion. What is considered moral or immoral has changed
dramatically in the last century alone. At some point in the future, our
morality will be as unrecognisable to future generations as Ancient Greek
morality is to us.
[1] Historically, this sort of moral absolutism was the impetus for certain
missionary activity. No not that kind.