Ahab was a bad king of Israel. Elijah was one of God's prophets.
One day Elijah went to see King Ahab. Elijah told King Ahab that the Lord God of Israel had said that no rain or dew would fall the next few years unless Elijah commanded it. God knew that Ahab would be mad at Elijah, so God hid Elijah near the Jordan River. He sent ravens with food to feed Elijah every day.
Just as Elijah had said, it didn't rain in Israel for three years.
Finally God told Elijah, "Go and meet King Ahab. I will soon send rain." So Elijah told Ahab to bring the 450 prophets of the false god Baal to Mount Carmel for a contest. The prophets of Baal built an altar and put a dead bull on it. They called on Baal to set the offering on fire. But nothing happened.
Then Elijah built an altar and put an offering of a dead bull on it. He poured 12 huge jars of water on his sacrifice. Elijah asked God to set it on fire. God sent fire from heaven to burn up the bull, the altar, and even the stones! Then all the people worshiped God.