What is the meaning of form in terms of design?

Kim Eun Hak's Pile-up series begins with this question.

Form is an essential requisite of design. Also, it is the first part that visually encounters the viewers. Therefore Pile-up series attempts to communicate with the user throught the factor form, which possesses the strongest visual influence.

Pile-up series has no fixed shape. The user chooses units and creates his own form by "piling up", and by this act each form created harmonizes with each other.

Furthermore the result of this interaction is conversation between the designer and the user.