Not sure about the "structural design" part (probably involves designing modelling systems), but I can explain the "optimal" part.
Optimisation is about getting the most out of something within a given set of constraints. A simple example is maximising profit, given a limited number of resources. Assuming non-linear systems, one way to solve them is using calculus (partial differentiation) involving Lagrange Multipliers, Kuhn Tucker conditions etc.
So my guess is "optimal structural design" is about creating modelling systems (often using programming) which optimise (usually maximise or minimise) a certain variable or a number of variables.
I could be way off though...lol