The People's Republic of China (PRC), governing mainland China including Hainan, Hong Kong, and Macau, has wanted to acquire control of the territory of the Republic of China (ROC), governing the islands of Taiwan, Penghu, Kinmen, and Matsu, for many decades. This acquisition process might happen if the ROC makes a clear statement of independence from the PRC, and the PRC invades in response and succeeds. In May 2020, PRC General Li Zuocheng said “If the possibility for peaceful reunification is lost, the people’s armed forces will, with the whole nation, including the people of Taiwan, take all necessary steps to resolutely smash any separatist plots or actions.” Foreign Policy thinks “Beijing is unlikely to invade Taiwan during the pandemic”, which raises the question of longer timespans. There are questions as to if/when China would be able to win a military conflict, or whether the US would defend Taiwan.

Alternatively, the ROC may voluntarily choose to unify with the PRC. Currently, however, the majority of the Taiwanese public is in favor of maintaining the "status quo"—i.e., is against unification with China. Over 70% believes that the Republic of China is an independent country. 6.7% believes that Taiwan and China should be unified as soon as possible (The Diplomat).

For further reading on this topic, see:

• Journal of Indo-Pacific Affairs, Air University Press: The Ambitious Dragon: Beijing’s Calculus for Invading Taiwan by 2030
• Council on Foreign Relations: Why China Would Struggle to Invade Taiwan
• Defense News: How DC became obsessed with a potential 2027 Chinese invasion of Taiwan

This question is based on @michaelchen's question 'Will the People's Republic of China control at least half of Taiwan before 2050?'