Rashi on Leviticus 16:2/רש"י על ויקרא ט״ז:ב׳

1יבא‎ ויאמר ה׳ אל משה דבר אל אהרן אחיך ואל AND THE LORD SPOKE UNTO MOSES, SPEAK UNTO AARON THY BROTHER THAT HE COME NOT [AT ALL TIMES INTO THE HOLY PLACE]! THAT HE DIE NOT as his sons have died (Sifra, Acharei Mot, Section 1 4).

2ולא ימות THAT HE DIE NOT — for if he comes into the Holy of Holies at any time other than Yom-Kippur he will die (Sifra, Acharei Mot, Section 1 4).

3כי בענן אראה means, for I constantly show Myself there with My pillar of cloud, and because the revelation of My Shechinah takes place there he should take care not to make it his habit to come there. This is the literal meaning of the verse. The Halachic explanation is: He shall not come into the Holy of Holies except with (i.e. on the occasion when he is going to raise) a cloud of incense on the Day of Atonement (Yoma 53a).