This means that they did not attain the Returning Light (the clothing for the Light [Ohr] of Hochma). Clothing is what raises a person from an animal level to the human level.
They didn’t have a Masach (screen); there was no Returning Light and there was nothing corresponding to the level they were on. Their mistake indicates that it isn’t easy for a person to reveal such a big ego.
By the way, these are special souls, so why didn’t they understand that they couldn’t do anything without clothing, without dressing in the Returning Light in the way that is required for that level? This is guidance from above. Here the Creator shows that He carries out His plan.
This is how new attributes, which we will later have to correct, are revealed in our world. They are always revealed in the form of wars, problems, plagues, and natural disasters after which there is gradual correction.
They didn’t wear the clothing to which bells were sewn, the sound of which rose the great priest to the recognition of the presence of the Almighty.
Bells were sewn to the edge of the clothing of the great priest (Cohen), which symbolized his lowest level. They symbolized the attribute of bestowal and created a sound that rose and spread in every direction.
This means that the attribute of bestowal stemmed from the clothing of the priest (Cohen) in which his ego was dressed. The ego was so corrected that the sound symbolized the spread of bestowal to the next level.
But Nadav and Avihu couldn’t be dressed in that clothing because they had to ascend through Aaron by connecting to him, but they didn’t do so.
This is basically the essential condition in order to ascend to the next level. The cloak with the ringing bells was essential in order to ascend from the level of a priest, which means the dressing in the attribute of bestowal.
From KabTV’s “Secrets of the Eternal Book” 1/15/14