what is a Pareto-optimal design?
You might have a look e.g. at Fig. 12 in the paper ηρ-Pareto Optimization and Comparative Evaluation of Inverter Concepts considered for the GOOGLE Little Box Challenge. 

What they do is the following: You select parameters of a converter system optimization, e.g. efficiency and power density (which characterizes the converter size), and you plot a point in the diagram for a specific design. You do this for a large variation on design parameters and get a lot of such points. The "outer border curve" of such a cloud of points, each representing a valid design based on different parameters, is called the Pareto front. 

It allows to conveniently select a good compromise of optimized designs concerning the target parameters, e.g. efficiency and power density. Other popular target parameters for power converter optimization are e.g. cost and/or weight. You can apply Patreto Fronts for all kind of design optimizations, not only power electronics.