Jamieson, Fausset and Brown on Leviticus 16:2

2. Speak unto Aaron thy brother, that he come not at all times into the holy place within the veil, &c.--Common priests went every day into the part of the sanctuary without the veil to burn incense on the golden altar. But none except the high priest was allowed to enter within the veil, and that only once a year with the greatest care and solemnity. This arrangement was evidently designed to inspire a reverence for the most holy place, and the precaution was necessary at a time when the presence of God was indicated by sensible symbols, the impression of which might have been diminished or lost by daily and familiar observation.

I will appear in the cloud--that is, the smoke of the incense which the high priest burnt on his yearly entrance into the most holy place: and this was the cloud which at that time covered the mercy seat.