Something that's creaky makes a groaning or scraping sound. If your front door is creaky, its hinges might need to be oiled.
The sound of footsteps on a creaky wood floor, or climbing your creaky stairs, can be spooky at night. You can also describe a hoarse or high-pitched voice as creaky, and your grandfather might sigh, "Oh, these creaky old knees," when he stands up. Creaky describes the grating sound, and also the worn out or run-down nature of old things or people. It comes from creak, which is imitative — its sound suggests its meaning.