I suppose you could be forgiven for not knowing, since you were only one years old, and in Germany, when it happened.
If you don't want a history lesson, to put it simply, in Mahy 1989, many Chinese students were protesting at the Tiananmen Square (in Beijing). They were protesting against a one-party dictatorship, corruption and a bunch of other stuff. They stayed at the square, organized demonstrations etc. By June fourth of that year, the government (a.k.a Communtist Party of China) decided that they had enough, and took military action. Troops were sent to the Tiananmen Square to get rid of the students. Many were jailed, some were killed etc. And to this day, the Communist Party refuses to speak of it. Most people in China have conveniantly "forgotten" it. A search on the internet (from Mainland China) would show almost no results. Even the Wikipedia page on the June Fourth incident is blocked.
So each year in Hong Kong there's a gathering, to remember those who died, or where unfairly treated. And to demand the Chinese government to face the facts. We also like to throw in a demand for democracy, even if that's as impossible as the 1000,000 posts thread.