A covenant is a solemn agreement. The most important covenants in the Bible are those in which God makes a covenant with man: the Abrahamic Covenant, the Davidic Covenant, etc. Here is a covenant which man makes with God. The Israelites recognize the importance of God and covenant together to wholly follow Him.
The Jews had been taken captive because of their disobedience. Here, they return to the Promised Land and begin to rebuild the city of Jerusalem. They begin with the Temple—the place of worship. Later, Ezra restores spiritual purity to their obedience. Finally, Nehemiah rebuilds the walls for protection.
