After his election victory, President Trump was openly challenging the idea of the “one China” policy, saying it should be entirely conditioned on getting better trade deals out of China, a policy which had been in place for generations, and which since 1979 has seen the US having no official diplomatic ties with Taiwan.
Today, however, Trump appears to have totally reversed overnight, talking with Chinese President Xi Jinping on the phone and assuring him of his intention to continue honoring the “one China” policy, which Chinese media are presenting as a huge victory.
As a matter of policy, “one China” means the United States only recognizes and has diplomatic relations with one China at a time. In the post-WW2 era, this meant only relations with Taiwan, though in 1979, when China realigned against the Soviet Union, Jimmy Carter revised the policy to recognize mainland China as the “one China,” cutting ties with Taiwan.
Since then, US ties with Taiwan have all been managed by non-profit organizations run by the State Department. In practice, Taiwan has a US Ambassador in everything but name, and an embassy that is simply listed as not an embassy for the sake of the policy. The US also continues to provide substantial military aid to Taiwan annually.