Matthew Henry Commentary Numbers 11:1-3

"And the people were as murmurers, [speaking] evil in the ears of Jehovah: and when Jehovah heard it, his anger was kindled; and the fire of Jehovah burnt among them, and devoured in the uttermost part of the camp. And the people cried unto Moses; and Moses prayed unto Jehovah, and the fire abated. And the name of that place was called Taberah, because the fire of Jehovah burnt among them." — Numbers 11:1-3 (ASV)

Here is the people's sin: they complained. Observe the sinfulness of sin, which uses the commandment itself as an occasion to be rebellious. The weakness of the law revealed sin but could not destroy it; it restrained sin but could not conquer it.

They complained. Those who have a discontented spirit will always find something to quarrel or worry about, even if their outward circumstances are very favorable. The Lord heard their complaining, though Moses did not. God knows the secret anxieties and murmurings of the heart, even when they are hidden from other people.

What God observed, He was very displeased with, and He chastised them for this sin. The fire of their wrath against God burned in their minds; justly, then, did the fire of God's wrath take hold of their bodies. But God's judgments came upon them gradually, so that they might take warning.

It became clear that God does not delight in punishing; when He begins, He is soon persuaded to relent.