What does "squeaky wheel gets the grease" mean?

The loudest complaints get the most attention, as in No matter what table they give her, Helen generally insists on a better one and gets it—the squeaky wheel gets the grease. The current version of this idiom, with its allusion to a wagon wheel that needs oiling, is ascribed to American humorist Josh Billings (1818-1885) in a poem, “The Kicker”: “I hate to be a kicker [complainer], I always long for peace, But the wheel that does the squeaking Is the one that gets the grease.” However, the idea of the idiom is much older. A manuscript from about 1400 had: “Ever the worst spoke of the cart creaks.” Similar sayings were repeated over the succeeding centuries.