Affordances are measured relative to the observer (will that board hold me up? Can I fit through that opening? Can I sit on this?). We see things in the world according to our purposes, and our purposes are not always reflective or conceptual purposes. Our bodies know what they need to sit on, what they need to fit into, what they need to fear, and so forth, and we use concepts when there is some question or need for reflective ability. It means that we do not simply see the existence and properties of a thing, but also the meaning or value of a thing. This is not after the fact, but part of the perception itself.
"The affordances of the environment are what it offers the animal, what it provides or furnishes, either for good or ill. The verb to afford is found in the dictionary, but the noun affordance is not. I have made it up. I mean by it something that refers to both the environment and the animal in a way that no existing term does. It implies the complementarity of the animal and the environment...
Gibson, James. J. The Ecological Approach to Visual Perception. 1979. (p. 127)
There are three fundamental properties of an affordance:
Defined in this way, affordances cut across the subjective/objective barrier. They are objective in that their existence does not depend on value, meaning, or interpretation. Yet they are subjective in that an actor is needed as a frame of reference. By cutting across the subjective/objective barrier, Gibsonís affordances introduce the idea of the actor-environment mutuality; the actor and the environment make an inseparable pair.
Joanna McGrenere & Wayne Ho, "Affordances: Clarifying and Evolving a Concept" http://www.dgp.toronto.edu/~joanna/papers/gi_2000_affordances.pdf