Israel grumbled (again) and God burned some of them to death with fire.
Just as when they left Egypt, Israel quickly began complaining after they left Sinai. GOD heard:
1 Now the people were grumbling evilly before the Lord, and the Lord heard them and was provoked to anger. So the fire of the Lord burned among them and utterly destroyed part of the camp. 2 Then the people cried out to Moses, and Moses prayed to the Lord; and the fire was quenched. 3 So he called the name of the place Burning, because fire from the Lord burned among them.
(Numbers 11:1-3 OSB)
The whining of Israel was not a first-time event. We do not know what Israel said here in their evil grumbling, but we know that GOD is patient and long-suffering and He was greatly angered by their actions.
GOD's dealings with Israel's multiple rounds of complaining teaches us about progressive discipline. GOD was merciful the first time, providing manna. However, as the whining continued, GOD became increasingly firm in delivering the message to the people that He was providing for their needs. Rather than taking their concerns to GOD, they just complained and griped about their circumstances.