Optimal design means making things in the best and most efficient way so that they work really well.
For example, let's say we are going to build a car. To make the best car, we need to have a plan, or a design, that takes into account all the things that the car needs to do, like go fast, be comfortable, and not use too much gas.
To do this, we need to think about things like where to put the wheels, how to shape the car, and what kind of engine to use.
If we make good choices about all these things, we end up with a great design for our car that works really well and can do all the things we want it to do. That's what it means to have an optimal design – a design that is just right for what we want to use it for.