Short answers, pulled from the story.
The Xi'an Incident was a political crisis in China that lasted from the 12th to the 26th of December 1936. Generals Zhang Xueliang and Yang Hucheng kidnapped Nationalist leader Chiang Kai-shek in Xi'an to force him to end the Chinese Civil War and ally with the Communist Party against Japan.
Zhang Xueliang had lost his home province of Manchuria to Japan and deeply resented Chiang's policy of fighting Communists rather than resisting Japanese expansionism. After Chiang threatened to remove Zhang from command on the 9th of December 1936 if he refused to attack the Communists, Zhang ordered the arrest two days later.
Stalin overruled the CCP's initial call for Chiang's execution, fearing the coup was a Japanese plot to weaken China. He was also alarmed that Chiang's likely successor, Wang Jingwei, had recently met with Adolf Hitler to discuss China joining the Anti-Comintern Pact. The Comintern sent a telegram on the 16th of December ordering a peaceful resolution.
Zhang Xueliang voluntarily accompanied Chiang Kai-shek back to Nanjing on the 25th of December 1936 and was immediately arrested. He was court-martialed on the 31st of December, sentenced to ten years in prison, then held under house arrest for over 50 years, until 1990.
Zhang Xueliang and Yang Hucheng sent telegrams to Nanjing on the morning of the 12th of December 1936 listing eight demands, centered on ending the civil war against the Communists and forming a united front to resist Japan. Chiang never signed a formal written agreement, but verbally conceded to key conditions before his release on the 25th of December.
The Xi'an Incident produced a ceasefire between the Nationalist government and the CCP and led to the formation of the Second United Front, formally enacted in late September 1937 after the Second Sino-Japanese War had already begun. Nationalist historians argued it saved the CCP from destruction; some revisionist historians contend the basic terms were already taking shape before the coup occurred.