Good has various meanings depending on one's perspective. Humanists typically define good as any action or lack of action that causes an improvement in the life, health, happiness, or quality of humans or society. Many theists define good as an action which is in accordance with God's will.
Some theists will even claim that the word "good" is related to the word "God", but in fact they have two completely different etymologies. "Good" comes from the Proto-Germanic word *gothaz, while "God" comes from the Proto-Germanic word *guthan.
The concept of "good" and God's will
The theistic definition of good given above is problematic. If good is defined as whatever is in accordance with God's will then there is nothing to stop God from declaring murder and rape "good". If murder and rape are God's will then they must be good. Furthermore, what makes God's will automatically good? Does he get his definition of good from another source? If so, where does the source get their definition of good? If not, then he's really just making up the definition of good as he goes along.
This issue is further explored by the Euthyphro dilemma.