Affordances – the relationship between a physical object and a person. It is like the property with its own usage. Anti affordances (constrain) – the prevention of interaction.

Signifiers – ways to communicate where action should take place .

Below is a great example to illustrate what affordances and signifiers are.

Constrains of the lamps above are the on button, which is the only way to on the light. It prevents user from second guess the whole surface works as a on button. The flat surface at the bottom indicates that it is only way to have the lamp stand still or properly.