Computational design is not any one algorithm or off-the-shelf process you can apply. Rather, we describe it as an approach whereby a designer defines a series of instructions, rules and relationships that precisely identify the steps necessary to achieve a proposed design and its resulting data or geometry.
Crucially, these steps must be computable, meaning they can be understood and calculated by a computer.
Above: Image of an NURBS manipulations from Martin Stacey - UCL.
Put simply, computers are very good at performing calculations and executing pre-defined steps.
When approaching a design computationally, the designer would focus on developing the procedure that would create a design - not the design itself. The process of iterating through options and data are offloaded to a computer. This saves time, money and effort, and lets the designer focus on the creativity of the design process.