Let’s say ethics is a set of mappings from possible actions to one of three values: permitted, forbidden, and optional.
An ethics is absolute if it holds that this function is invariant across times, places, and cultures.
An ethics is relative if it claims that these functions are in turn a function of time, place, and culture.
So an absolutist would claim either “Killing a spouse taken in adultery” is forbidden, whatever the society, or it is always permitted.
However the relativist could argue “In societies where revenge and blood feuds are a recent memory killing a spouse taken in adultery is permitted. However in societies where revenge and blood feuds are under control killing a spouse taken in adultery is forbidden.”
However those functions are not in turn relative. The relativist believes the rule that carries us from society to moral rule is itself absolute.