The LORD said to Moses: “Tell your brother Aaron that he is not to come whenever he chooses into the Most Holy Place behind the curtain in front of the atonement cover on the ark, or else he will die. For I will appear in the cloud over the atonement cover.
Or "purification offering" ; here and throughout this chapter
He is to put on the sacred linen tunic, with linen undergarments next to his body; he is to tie the linen sash around him and put on the linen turban. These are sacred garments; so he must bathe himself with water before he puts them on.