What does Leviticus 16:2 mean?

The Lord told Moses to tell Aaron not to come into the inner sanctuary behind the curtain, in front of the cover on the Ark, whenever he wants, or he'll die. That's where I appear in a cloud above the cover. Leviticus 16:2 - Modern Text Bible

(God warns Aaron that entering the holiest place is dangerous and can't be done casually, because God's presence is there.)