Take yourself over to the Center for Responsible Nanotechnology for a good introduction to what is meant by molecular manufacturing. You might read it as "advanced nanotechnology":
Molecular manufacturing is the use of programmable chemistry to build exponential manufacturing systems and high-performance products.
Today's nanotechnology is should be viewed as early nanoscale engineering: it represents the first successful attempts to commercialize and mass-produce products with deliberately designed features of nanoscale lengths. The vision of a mature molecular manufacturing technology base bears much the same relationship to the nanotechnology of today as present day mass-production technology bears to the early innovations in factory technology in the 1800s. In other words: vastly more, better and cheaper techniques taking place in a world of far greater general technological capabilities.
That world is not too far off.
One consequence of mature molecular manufacturing is that, in concept, you can pretty much make anything for the cost of raw materials, a blueprint, and the time it takes to assemble your product. Most items of common usage can be made from raw materials that are essentially free - the component elements of soil and rock. This is a powerful economic liberation - that everything becomes a matter of information: the cost of the blueprint and learning to use the product you just built.
In order to get to this point, we will have had to reach a couple of quite important technological milestones. The first being to build systems that can identify, manage and place trillions of molecules accurately. The second to solve all the problems of computational capacity and complexity management required for the first goal. Once we have that, the sky is the limit; these milestones are important for what they imply about our technological capabiilties outside the realm of molecular manufacturing - such as our ability to understand and control our own biology.
Systems that can identify, manage and place trillions of molecules accurately are not a pipe dream; after all, we are already surrounded by examples. You, for example, are just such a system, albeit somewhat slow at self-assembly to full size. There's nothing in the laws of physics that jumps out and says we can't do this. It's just a matter of time.
If you have the technology base to build a nanoforge to assemble a brick, then you also have the technology base capable of simultaneously assembling and controlling a hundred million medical nanorobots of arbitrary design and programming. Or an artifical lung better than the real thing, or replacements for immune cells that never get old or worn. You get the idea. A brick is just as complex as any portion of the human body if you have to build the thing molecule by molecule; more fault-tolerant, but just as complex.
The open question, while we work away at early healthy life extension technologies like SENS during the next few decades - taking advantage of the biological nanomachines all around us, learning how to better repair, use and engineer them to extend our lives - is how close molecular manufacturing is to reality and then maturity. How long does it take to go from simple nanoforge and a proof-of-principle gram of assembled brick to the understanding needed to build a blood substitute a hundred times better than the old human 1.0 version and invulnerable to disease? What are the steps along the way? What will the business cycles and industries look like?
In the long run, when will we be able to build machinery that does the work of our biological components but is much better and far cheaper? When will we stop adapting and manipulating biology and start building the improved version?
We'd all like to live to find out; should we get there, aging will be something of a moot point, a fault in our biology that can be engineered out should we so choose. All the more reason to work harder on healthy life extension now.