Preliminary reports, like this at Nasdaq.com, say China's PLA has moved 150,000 troops to the North Korean border. It isn't clear what this means Beijing's decision-makers are thinking/fearing. I see three possibilities, one positive, one negative, and a third ambivalent.
First, they may believe the U.S. plans do something which will cause the collapse of the Kim Jong Un regime. This would potentially send a wave of starving Norks toward the Chinese border, a wave they intend to forestall. This is the positive implication of those troops' location.
The second implication is they are there to reinforce the North's army as they did during the Korean war 60+ years ago, in the event war breaks out on the peninsula. The troop positioning may be meant to give the U.S. a reason not to act. This is the negative implication.
Finally, it is entirely possible that the PLA is unclear which outcome will prevail but intends to be ready to deal with either outcome. China is clearly entitled to position the PLA anywhere within China they choose and Korea looks iffy at present.
It makes me wonder what Xi concluded from his recent meeting with Trump at Mar a Lago, what Trump conveyed intentionally or otherwise?