I'm definitely not the most qualified person to answer this but I'll take a crack at it. Everything I say here is my best understanding, and I'm sure some things are wrong, so - grain of salt.
The US debt China owns is in the form of treasury bonds issued by the US Treasury Department so the federal government can borrow money. Essentially, you buy a bond for $x, the US government agrees to repay you $x+interest over some period of time (10 years or so). The bondholder can sell the bond to somebody else, but they can't really "cash it in" early - it just gets paid back on a regular schedule.
What China can do (and in fact I saw a headline they did this today) is stop buying US treasury bonds altogether. Traditionally they've been one of the absolute safest and stablest places for investors to park money. Lower returns than stocks, etc, but with basically no risk. That stability is being threatened with all of this nonsense, as "the full faith and credit of the United States" isn't looking so hot anymore. Bonds are one mechanism that let us get by with all of the deficit spending we do, and in turn we can continue to issue debt and not have to reckon with our imbalanced budget. Not being able to continue to accrue debt probably doesn't sound like a bad thing, and I'm certainly not opposed to trying to lower our debt and reduce the annual deficit (though I'm not sure it needs to get to net zero). But what is bad is if the ability to borrow is suddenly yanked away or severely reduced, which would throw the whole system into (even more) chaos.
Looking it up now and China only owns ~2.6% of US debt, so maybe the effect of them pulling out of buying treasury bonds isn't a big deal. That's definitely not something I'm qualified to guess on.