that world markets are in retreat".
"That world markets are in retreat after Beijing cut the country's growth target to an eight-year low [subject] says [verb]investors' faith in China as the global economic saviour [complement]"
"It says something ..." means "There is some significance in the fact ..."
For a simpler example: "It says something about your personality that you beat your horse."
This means, "The fact that you beat your horse says something about your personality." And that 'something' is possibly that you are a callous, unfeeling person.
Student or Learner