Always present, always in the background; like a vigilant mother, the cloud had been watching over them, protecting them..
As the book of Shmot draws to an end, a cloud envelops and fills the newly completed Mishkan. In a general sense, we understand that this is what makes the Mishkan operational:
The cloud covered the Tent of Meeting, and God's glory filled the Mishkan. Moshe could not come into the Tent of Meeting because the cloud rested on it and God's glory filled the Mishkan. [Later], when [God] raised the cloud up from the Mishkan, it [would be a signal] for the Israelites to move on, [and this was true] in all their travels. When the cloud did not rise, they would not move on, [waiting] until the day it did. God'scloud would remain on the Mishkan by day, and fire was in it by night. This was visible to the entire House of Israel, in all their travels. (Shmot 40:34-38)
What is the significance of this cloud? Although we may not have paid proper attention to it, we have seen this cloud but not really focused on it throughout the entire book of Shmot; we might say that the cloud has been a major subtext. Thus, when the Jews first left Egypt, the cloud accompanied them:
God went before them by day with a pillar of cloud, to guide them along the way. By night it appeared as a pillar of fire, providing them with light. They could thus travel day and night. The pillar of cloud by day and the pillar of fire at night never left [their position] in front of the people. (Shmot 13:21-22)
Always present, always in the background; like a vigilant mother, the cloud had been watching over them, protecting them. Although we generally think of clouds as ethereal, as a bit of heaven, the cloud had been their constant companion in a very real sense, separating between their camp and the Egyptian army, leading them through the sea, showing them the way forward. In a similarly “real” sense, when the time came for the Revelation at Sinai, God descended to earth, as it were, and appeared on the mountain in a cloud:
God said to Moshe, 'I will come to you in a thick cloud, so that all the people will hear when I speak to you. They will then believe in you forever.' (Shmot 19:9)
When Moshe was invited to ascend Mount Sinai to receive the Tablets, he climbed heavenward, making his way through the cloud:
As Moshe climbed the mountain, the cloud covered the mountain. God's glory rested on Mount Sinai, and it was covered by the cloud for six days. On the seventh day, He called to Moshe from the midst of the cloud. To the Israelites, the appearance of God's glory on the mountain top was like a devouring flame. Moshe went into the cloud, and climbed to the mountain top. Moshe remained on the mountain for forty days and forty nights. (Shmot 24:15-18)
These verses, describing the Revelation, bear a striking resemblance to the verses at the end of the book of Shmot that describe the completion of the Mishkan. Perhaps by considering the similarities between these two sets of verses, we may gain a better understanding of the final chord sounded as Shmot comes to an end, and, as a result, of the Mishkan itself.
The key, it seems, is the cloud: According to tradition, the protective cloud that had accompanied the Israelites out of Egypt dissipated when the people worshipped the golden calf. This seems to represent a strange, stern quid pro quo: The people were confused; they felt vulnerable and abandoned due to Moshe’s absence, and they failed to appreciate that God’s Presence was still very much with them in the form of the protective cloud. And because they turned a blind eye toward the ever-present manifestation of God, taking the cloud for granted, it was taken from them. This is the price to be paid for not appreciating God’s protection: The protection is revoked. The cloud vanishes.
In the aftermath of the sin, Moshe prays for forgiveness on behalf of the nation. He pleads that God’s presence return and dwell among the people. Moshe goes so far as to say that if God is not in their midst He may as well not go through the motions of allowing the Jews to continue their journey to the Promised Land (Shmot 33:15-16). Moshe understood that without God in their midst, their efforts would be futile, meaningless.
This, then, is the true significance of the final verses of Shmot: The cloud has returned. For the first time, the people are granted a clear sign that the sin perpetrated at the foot of the mountain, the sin that had banished the cloud, has been forgiven. The cloud expresses the rekindled intimacy between the Jewish People and God. Now that they are once again granted protection and guidance, they are able to move on, both spiritually and geographically, continuing their quest to create a holy society in the Holy Land.
For a more in-depth analysis see: http://arikahn.blogspot.co.il/2016/03/audio-and-essays-parashat-pikudei.html