I would like to add to the existing answers, and explain a bit more why this visit is perceived as being so significant, by considering the following two strategic goals of China:
Pursuing closer ties with Eastern Europe
China has been actively pursuing closer ties with Eastern Europe. There are economic and geopolitical reasons for them to do so, in particular, it is not in their interest for Europe to be firmly in the U.S.'s sphere of influence (in that regard their goals align closely with Russia's). Up until about 2018, the general trend was that they were succeeding. However, the economic rewards for countries like the Czech Republic somewhat failed to materialize, and relations have subsequently failed to blossom. For an Eastern European nation to undergo such a reversal is embarrassing, but for them to actively pursue relations with Taiwan is far too much for them to tolerate.
Increasing Taiwan's isolation
Even more significant is that the trend of continuing international isolation of Taiwan has recently stagnated, if not reached an inflection point. China is open about their long-term strategy to re-unification of their historical territory. Deng Xiaoping himself said:
Reunification of the motherland is the aspiration of the whole nation.
If it cannot be accomplished in 100 years, it will be in 1,000 years.
China regards their political system as superior to the democratic systems in this regard, because they are, in principle, able to sustain diplomatic pressure over very long timescales. Democratic systems struggle to be so consistent, due to the ebb and flow of domestic politics. The COVID pandemic, and China's response to it, has if anything worsened this trend. No amount of time will allow their approach to succeed if the trend is going in the wrong direction. This also explains why they acted to forcibly take control of Hong Kong, and why they have significantly stepped up their military pressure on Taiwan.
Plenty of national leaders will go to extreme lengths to avoid the appearance of failure. In China's case, failure on issues of national rejuvenation is simply not an option.