Their slogans' English is a bit... never mind. Anyway, what is happening is that the HK government is proposing the copyright amendment bill. This will become an easy tool for the regime to suppress free speech and arrest political dissidents, because activists often use and distribute derived materials for satirical purposes. The most ridiculous part of the bill is that it allows the government to bypass the copyright holder to make criminal prosecutions against individuals, even if the copyright holder does not mind the use of their work.
The US government is supporting the bill for the commercial interests of US companies, and either is unaware of, or more likely, couldn't care less, about its political consequences. Seen in pictures are a "localist" political group ("HK-as-a-city-state") who have long advocated that the mainstream pro-democracy camp in HK are just working for the US, and not for HK people. This incident adds new evidence to this claim: those mainstream political parties pretend to be against the bill (so as to be seen on the side of free speech etc), but will do nothing other than sitting there and casting their votes, knowing full well that they don't have enough votes to stop the bill going through (due to the way the system is designed to prevent any real democracy.) Hence this localist group is protesting here directly to the US, appealing it to cease its support.