Design-ahead is the idea that we can design things that cannot be built yet. The products of future nanotechnology will follow the laws of physics, just as horseshoes and jet engines do. Those laws are knowable today, and they allow us to reason about future gadgets that we can't yet make.
Using hammers and tongs and a forge to make metal hot and soft, a blacksmith can make a horseshoe. He can make an axe or a sword or metal parts for a wagon. But he can't make a jet engine. The reason he can't make a jet engine is because the necessary tolerances are much too precise. Building a jet engine takes a more advanced form of manufacturing technology than blacksmithing.
The blacksmith can read about thermodynamics and materials and fluid mechanics and other sciences, and he can start to reason about whether a jet engine could actually work. Given a jet engine design, he can calculate how strong the metal needs to be, the pressure and temperature of gasses flowing through the engine, how much thrust it could deliver, and other such things. He can determine whether a design is theoretically feasible or theoretically disallowed by physical laws, even if he can't build the engine.
Many indicators suggest that within a few decades, we will be building machines of molecular size. These machines will have moving parts: gears, axles, bearings, rods, pistons, all the machine parts we are familiar with today, in addition to much smaller versions of today's electronics. We will be able to fit hundreds of millions of moving parts inside a small fraction of the volume of a human cell. These can be used to build machines that monitor the cell's health and protect it from viruses and some effects of ageing. We will also be able to make much stronger materials than we can make today, because we'll make large pieces with no material flaws in them.
Why bother with design-ahead? For a few reasons. One is that it will help us get to the point of really doing this stuff. Another is that it will help us plan for things that can go wrong, like nanotech weapons getting into the hands of terrorists and rogue states. Another is to encourage people to learn about nanotech so that the economic disruption is mitigated when it arrives. The kids who study physics and chemistry and computers today can be the designers of tomorrow's nanotechnological gadgets.