The basic idea is that in any outbreak of an infectious disease with high transmissibility the initial growth tends to accelerate into exponential increase--a very scary thing. That's how you could theoretically wind up with One Billion Infected in 81 Days. However, that exponential increase tends to run up against a number of countervailing factors. One obvious example is that once a given population is saturated, the rate of infection must slow--even drastically. That event--when exponential rate of increase slows--is the "inflection point" when the exponential curve transforms to a "logistic curve": logistical considerations take over.
This can happen naturally, when the population in question is somewhat isolated. That's not the case with China--Chinese travel the world, and we're seeing the results of that in a now-declared pandemic in which centers of infection are thousands of miles apart. However, this transformation of the exponential curve can also be induced by human action, especially by social isolation--removing potential new subjects of infection from the risk zone. Travel restrictions are another obvious way to create an inflection point, as well as medical treatment. There are other considerations, but it should be obvious that it's very much in our interests to induce an inflection point as quickly as possible.
When the Chinese authorities saw--in late December--that the new virus they had first identified in mid-November was accelerating into exponential growth they scrambled and adopted draconian measures to induce an inflection point, which they now claim to have reached. Every country that has had some success in combatting Covid-19 has adopted some version of that approach--introduce drastic measures to induce an inflection point that will break the exponential rate of increase and prevent a breakdown of the healthcare system. (For an account of the flaws in the Chinese response, see this very informative article from the SCMP: Coronavirus: China’s first confirmed Covid-19 case traced back to November 17.)
Thus the narrator concludes:
"If people are sufficiently worried, then there's a lot less to worry about. But if no one is worried, that's when you should worry."
Keep those words in mind when assessing our own government's response.