It means that God is perfect and that there is no sin in him at all. The same word (“good”) is used by God, before sin came into this world, when he said that the world was good as he had made it. It was perfect, without sin. You can also see how good Yahweh is when you look at what he does. God himself says that his goodness is who he is (see Exodus 33:19). That is his general characteristic. And what that goodness means is also revealed in all the attributes of God, that were mentioned when he proclaimed his name to Moses (Exodus 34:5–8).
8 Oh, taste and see that the LORD is good! Blessed is the man who takes refuge in him!