It is a rather simple concept, and I'm going to explain it in Christian terms:
God made the universe and all that dwell therein.
Thereby, God is Sovereign above all.
If God is Sovereign above all, then HIS definition is THE definition that matters, because His is the final say-so. If He says something is Good, then it is Good because of His Sovereignty.
There are none higher than Him, therefore nobody has the right to dispute anything He says. We're talking about a Being who spoke the entire universe into being, stars that are (to us) countless, yet He knows them each by name.
Those who disagree with Him and choose not to follow Him or seek Him will eventually be eternally separated from Him when they leave their mortal bodies. Since God is Life, He is Love, and He is the Light... being without Him leaves us with no Life, no Love, and no Light -- eternal darkness and anguish, aka, "Hell".
People think of Hell as His punishment... it is more like Him giving these people what they seem to want -- they don't want Him or His Ways, so they get separated from Him instead.