The Hebrews have passed the Red Sea but they need water. When they happen upon a watering place, the water is bitter, and the people murmur. God shows Moses a tree, much as God shows the people the law, and the water becomes sweet. God uses this to test the people to see if they will now walk in His ways.
Later, the people are hungry and murmur again. This time God provides them with mana and quail, but instructs the people to take only what they need for that day because God will provide. Those who stored up more than they needed for that day found that their food spoiled. Paul comments on this in 2 Corinthians 8:14.
The people murmured again desiring water, and Moses, fearing for his life, sought out the Lord, who told Moses to strike a rock with his rod in the presence of the elders, to bring forth water.
The Hebrews encounter the Amalekites, and fight them. Moses raises his hands in persistent prayer and the people help him to hold his hands up. Because of this, the Hebrews win in battle. God promises to blot out the Amalekites, and the people are called to remember this in Deuteronomy 25:17-19. This is further recalled in 1 Sam 15, and in Esther, Haman, the descendant of King Agag of the Amalekites plots against the Jews.