President Trump, before he took office, took a phone call from the president of Taiwan. It didn't go well. But in a call with the leader of China, Trump has now agreed to respect the "one China" policy.
In 1949, China split between the mainland, ruled by Communists, and Taiwan, ruled by Nationalists. In 1992, the two agreed that there was one China. In practice, that's meant China leaves Taiwan alone, while Taiwan doesn't try to declare formal independence.
The current ruling party in Taiwan, the DPP, wants to declare independence. That means they won't always abide by One-China niceties. For the U.S. government, that requires being careful about things like taking phone calls from the head of state in Taiwan (especially when the DPP is in power).
WHY IT MATTERS:
Trump used China as a punching bag during the campaign, saying they're stealing our jobs and tanking our economy. The phone call from Taiwan was an escalation of that. But if the Trump call with China has been reported accurately, it looks like even the Trump administration recognizes the benefits of respecting the status quo. And one reason that matters is because of the South China Sea.