The meaning is: "dream", "chimera".
See Primary/secondary quality distinction and Mechanism.
The idea is that our way to "see" and perceive the properties of things outside us is determined by the actions external objects elicit on our sense, but in no way we are licensed to infer from our perception that external objects are "exactly" as we perceive them.
"I think that tastes, odors, colors, and so on are no more than mere names so far as the object in which we locate them are concerned, and that they reside in consciousness." - Galileo Galilei, The Assayer (Il Saggiatore, 1623).
"[I]t must certainly be concluded regarding those things which, in external objects, we call by the names of light, color, odor, taste, sound, heat, cold, and of other tactile qualities, [...]; that we are not aware of their being anything other than various arrangements of the size, figure, and motions of the parts of these objects which make it possible for our nerves to move in various ways, and to excite in our soul all the various feelings which they produce there." - René Descartes, Principles of Philosophy (Principia philosophiae, 1644 - Les Principes de la Philosophie, 1647).