What does Genesis 8:20 mean?

Noah's first recorded act after leaving the ark is one of worship. He builds an altar to the Lord and offers animal sacrifices on it. This is the first time Scripture refers to building an altar to God. In the previous chapter, God sent seven pairs each of every kind of clean bird and animal. That was the first hint that God regards some animals as clean and others as unclean. Only clean animals could be used as sacrificial offerings to God (Leviticus 11; Deuteronomy 14).

Noah's act here corresponds with the most common form of offering to God, which Israel would later practice while following God's Law. In that offering, the whole animal is burned and fully consumed by fire on the altar. This offering would have been a truly faith-based sacrifice, even if it was commanded directly by God. So few of each kind of animal existed in the world that to purposely kill any of them, even the more plentiful clean animals, was very costly to Noah and his family. It was clearly an act of faith in God's ability to provide.

This act of worship to God reveals that Noah continued to be faithful to God, even after the flood. Noah proves that he is motivated by allegiance to God. As far as Noah was concerned, this new world remade by the flood would be built on a foundation of obedience and submission to the Creator.