Moral relativity is the wrongheaded idea that there is no absolute Right or Wrong, and that anyone can freely use his own conscience to decide what is moral. A moral relativist will not say that theft or murder is wrong, because he believes it is up to the murderer or thief to decide whether his behavior is justified.
Unsurprisingly, moral relativity is exclusively a liberal belief, as conservatives believe that God is the ultimate arbiter of Good and Evil:
"There is a way that seems right to a man, but in the end it leads to death." (Proverbs 14:12)
Moral relativity and related foolish thinking is what allows liberals to support abortion, gay rights, and drug abuse. Moral relativity erodes principled self-defense and thereby leads to misguided demands for gun control as well as psychiatric problems resulting from a lack of mental self-defense.
It could be argued that moral relativity allows one to understand other cultures and ways of life better, thereby avoiding unnecessary conflict.
The Theory of Relativity has generated a huge following by advocates of moral relativism. The idea of moral relativity exist independent of (and substantially predates) the theory of relativity, but some moral relativists irrelevantly invoke the theory in attempts to lend legitimacy to this version of morality.
Advocates of moral relativity seized on the theory of relativity to legitimize their views. Historians such as Paul Johnson wrote about how the theory of relativity caused a sea change, justified or not, in 20th century thought.