16:8
The provision of manna and meat was to be a rebuke against the Israelites, a rebuke of their grumbling. Moses then makes clear that their grumbling against him and Aaron was actually grumbling against God. In the provision of food God was demonstrating that He was well able to supply all their needs, and their grumblings were unfounded.
16:9-12
The people had grumbled against Moses and Aaron, though really against God. They were not to come before the Lord, the one they had really grumbled against. While they were saying this, the Lord appeared in a cloud of glory. God often appeared in a cloud. This would both demonstrate His presence, but also deny them full view of God. For if they saw God in fullness they would die. God spoke to Moses and told him what to say to the people. Moses was to repeat to them the provision of food that the Lord would make, and that it was in response to their grumblings.