Something formless either has no definite shape, like fog or smoke, or lacks structure, like a formless movie that doesn't seem to have much of a plot.
You can use this adjective to describe things that are physically vague and shapeless, from formless crowds of people surging forward at a rock concert or the formless, ghost-like shapes in your dark basement. It's also good for things like songs, poems, ideas, or concepts that are unstructured or incoherent and hard to make sense of.