What does a word like apple mean? This is really many questions in one.
How could hearing the word apple make you imagine an apple? How could seeing an apple activate a word-agent for apple? How could thinking about an apple make one think of the word for apple? How could seeing an apple make one wordlessly recall the flavor of an apple?
It's usually impossible to perfectly define a word because you cannot capture everything you mean in just a phrase; an apple means a thousand things. However, you can usually say some of what you mean by making lists of properties. For example, you could say that an apple is something round and red and good to eat. But what exactly is a property? Again, it's hard to define that idea — but there are several things to say about what properties we like our properties to have.
We like the kinds of properties that do not change capriciously.
The color of your car will stay the same from day to day, and, barring accidents, so will its basic size and shape, as well as the substances of which it is made. Now, suppose you were to paint that car a new color: its shape and size would remain the same. This suggests another thing we like to find in our properties:
The most useful sets of properties are those whose members do not interact too much.
This explains the universal popularity of that particular combination of properties: size, color, shape, and substance. Because these attributes scarcely interact at all with one another, you can put them together in any combination whatsoever, to make an object that is either large or small, red or green, wooden or glass, and having the shape of a sphere or of a cube. And we derive a wonderful power from representing things in terms of properties that do not interact: this makes imagination practical. It lets us anticipate what will happen when we invent new combinations and variations we've never seen before. For example, suppose that a certain object almost works for a certain job — except for being a bit too small; then you can imagine using a larger one. In the same way, you can imagine changing the color of a dress or its size, shape, or the fabric of which it's made, without altering any of its other properties.
Why is it so easy to imagine the effects of such changes? First, these properties reflect the nature of reality; when we change an object's color or shape, its other properties are usually left unchanged. However, that doesn't explain why such changes do not interact inside the mind. Why is it so easy to imagine a small brown wooden cube or a long red silk skirt? The simplest explanation is that we represent each of the properties of material, color, size and shape in separate agencies. Then those properties can simultaneously arouse separate partial states of mind at once, in several divisions of the mind. That way, a single word can activate many different kinds of thoughts at once! Thus the word apple can set your Color agency into a redness state, put your Shape agency into a roundness state — or, really, into a representation of an indented sphere with a stem — and cause your Taste and Size agencies to react in accord with memories of previous experiences with apples. How does language do such things?