After leaving Noah's Ark, Noah built an altar, according to Genesis 8:20. He then took some clean animals and clean birds and offered burnt offerings on it.
The altar is the first structure Genesis explicitly says Noah built after the Flood. Its location is not named in the passage. The act marks the transition from survival inside the Ark to worship on the restored earth, before the covenant and rainbow sign are described in Genesis 9.
The detail also connects with the earlier distinction between clean and unclean animals. Genesis says Noah brought extra representatives of clean animals and birds into the Ark, while the basic pairs of other animals were preserved. The text therefore links the post-Flood offering to instructions given before the waters arrived.