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 the resulting data or geometry. Crucially, these steps must be computable, meaning they can be understood and calculated by a computer.
Image above Martin Stacey - UCL - NURBS manipulation.
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, and not the design itself. The process of iterating through options and data are offloaded to a computer. This saves time, money, effort, and lets the designer focus on the creativity of the design process.