According to representative realism, we do not perceive objects directly. Rather, objects cause us to have certain experiences, sense-data, and it is these to which we have direct access. Representative realism thus introduces a distinction, not present in naive realism, between our experiences of objects and the objects themselves. John Locke was a leading advocate of this theory.
Consider colour properties. There are two different ways of thinking about colour. The first is in scientific terms: colour is to with reflecting certain wavelengths of light. The second is in experiential terms: colour is a subjective experience that a normal observer has when they look at a coloured object.
Primary and Secondary Qualities
Representative realism holds that there are two completely different types of property, corresponding to this distinction.
First, there are primary qualities, which objects have independent of any observer. An object is square, or heavy, for instance, irrespective of whether anyone is perceiving it to be such. Shape and weight are therefore primary qualities.
Second, there are secondary qualities, which objects only have because they are perceived. Secondary qualities, like colour, are projected onto the world by perceivers. The apple isn’t really that shade of red, it just appears that way to me; I project redness onto the apple in the act of perceiving it. Colour, then, is a secondary quality.
We tend to use secondary qualities to represent primary qualities. Thus the red appearance of the apple, which is a secondary quality that I project onto it, represents the fact that it reflects certain wavelengths of light, which is a primary quality that it has irrespective of whether I am looking at it or not.