The theory of value, also known as axiology, studies what kind of things and outcomes are morally good, or intrinsically valuable. This is in contrast to the theory of the right, which studies what people morally ought to do.
The theory of the good is typically aggregative - concerning some quantity which it is better to increase or decrease. However, in some cases people might also include principles (e.g. “an outcome is better the fewer rights violations it contains”).
There are many possible theories of value. Some hold that there is just one source of value, while others rely on multiple sources. Almost all theories agree that well-being (also known as “welfare”) has value, and some theories, known as welfarist theories, hold that it is the only thing which has intrinsic value. Non-welfarist theories recognize other sources of value, such as fairness, equality, or beauty.
Schroeder, Mark. 2016. Value theory. In Edward Zalta (ed.), Stanford Encyclopedia of Philosophy.