"On that day King Ahasuerus gave to Queen Esther the house of Haman, the enemy of the Jews. And Mordecai came before the king, for Esther had told what he was to her. 2 And the king took off his signet ring, which he had taken from Haman, and gave it to Mordecai. And Esther set Mordecai over the house of Haman."
When the kingdom of Israel continued to sin against God, he sent the Babylonians to conquer them, which they did in 587 BC. The king at the time, King Nebuchadnezzar, took the elite Hebrews from the nation of Judah and brought them back to Babylon so they could work for him.
The book of Esther takes place after the Babylonian exile had ended. Many of the Israelites stayed behind in Babylon, and Esther was one of them. The bad guy of the story, Haman, plots to have the Hebrews killed, but Esther and her uncle, Mordecai, foil the plan. Haman ends up executed while Mordecai becomes a nobleman.
King Ahasuerus gives Mordecai his signet ring as a symbol of Mordecai’s new position. Mordecai receives this great honor from the king and is elevated into a position of authority over the property of his enemy, Haman. God saved the Jews, and this ring is a symbol of that protection.