Parametric design generates a model from its parameters rather than its geometry. In other words, rather than using sculpting tools to make a 3D object in a computer modeling program, we use a program plugin that controls the underlying values for each operation (ie move, rotate, extrude, twist, scale) to exercise vast control over objects. This allows simultaneous and generative control over large amounts of objects in a single model. Generative in this case means that when one object changes, others that need to change in accordance will update automatically.
Question: How does this help a design project?
Answer: You can see more options for your project faster and make changes almost immediately by changing the values in your parametric definition, as opposed to manually manipulating the geometry of your model.
Architects and designers have been using parametric design software for the last fifteen years or so, but we have yet to build a platform for all people to access the creative wonders and efficiencies it provides in any design collaboration.