# built an altar to Yahweh

"built an altar dedicated to Yahweh" or "built an altar for worshiping Yahweh." He may have built it with stones.

# clean animals ... clean birds

Here "clean" means that God allowed these animals to be used in sacrifice. Some animals were not used for sacrifices and were called "unclean."

# offered burnt offerings

Noah killed the animals and then completely burned them up as an offering to God. Alternate translation: "burned the animals as offerings to Yahweh"