"Holy, holy, holy" is what the angels who hover about God sing eternally. The importance of this bit of song is that it illustrates the Christian relationship between God and man. The thinking is that, because God is neither nice nor mean, the angels sing that He is simply perfectly holy. It has a lot to do with the idea that God hates sin, but at the same time, loves the sinner. While He must judge those who do not follow Him, he still loves them because they are His creation.