Then the LORD rained on Sodom and Gomorrah sulfur and fire from the LORD out of heaven.
The people of Sodom and Gomorrah were very wicked. Their evil is described in the narrative of two angels (in the form of men) visiting and staying with Lot, the only righteous man in the place. While visiting, the locals attempt to capture the angels to do unspeakable things to them.
Lot and his family are saved while God destroys everyone else completely and utterly. The fire from heaven contrasts with the water of the recent flood account and shows that God is still angry with the wicked. Despite his mercy in not destroying the world by water again, God still judges.
The destruction of Sodom and Gomorrah is a sign that he will punish the wicked if they do not turn from their evil ways and follow God as Lot does in this account. The same applies to people today.