In OT times this was a powerful and wicked city, of about 120,000 people, situated about 960km (600mls) NE of Jerusalem. God told Jonah to go to this Assyrian city, whose inhabitants were enemies of the Jews, and warn of impending judgement unless they repented. In 760 BC Jonah arrived at Nineveh and the people responded to his message, repenting of their evil ways and so judgement did not come upon them at that time (Jnh 1:2, 3:2-10, 4:11; Jer 18:7-10). However, succeeding generations reverted again to evil. The book of Nahum outlines the impending doom of Nineveh, which had become the capital of Assyria (Nah 1:14, 3:7; Zeph 2:13-15). It was destroyed in 612 BC by the Medes.
Genuine repentance averts judgement. Is there some area of sin in my life that I need to turn from or am I prepared to suffer the consequences that will ultimately come if I continue in rebellion?