Good: What someone approves, desires, likes to exist.
In general good is a term of approval, often but not necessarily ethical or moral approval. And as defined, what is good clearly depends on the person who judges something.
The general problem here is well expressed by Sextus Empiricus, in "Against the Ethicists":
"..both layman and philosophers share the same pre-conception and believe that good and evil exist, - good being that which attracts them and is useful, and evil that which is of opposite nature, - but as to particular instances they are at war with one another: -
One thing is pleasing to one man, another thing to another,
and, in the words of Archilochus, -
Men differ as to what things cheer their hearts,
seeing that this man welcomes glory, that man wealth, another well-being, and another pleasure. And the same account applies to the philosophers." (p. 407)
This does not mean that, therefore, what is good is merely a matter of taste, since ethical and moral terms, and related uses of "good", normally involve quite complicated appraisals of human nature, human society, and human capacitities.
Also, this does not mean that what is good is utterly relative: If there is a human nature all humans share, and/or if there are a number of capacities all humans have or lack, then there is a factual basis that is relevant to many judgments of good and bad.
In Chapter 11 of "On "The Logic of Moral Discourse"" there is a minimalistic realistic treatment of "good" and "bad".