Basically, a nuclear reactor that begins with a thin slice of U235 - the fissile fuel used in "normal" reactors - and alot of U238, the non-fissile material from which U235 currently has to be extracted at great cost.
The small slice of U235 placed at one end of a cylinder of U238 is ignited in a nuclear sense, producing energy and also causing the U238 in proximity to it to become P239, Plutonium, which, being fissile itself, then continues the reaction, converting its U238 neighbours further along the cylinder still into P239, i.e. slowly burning its way through the cylindrical core like a cigarette. The beauty of this system is that it removes the need for bulk creation of U235 and uses what would otherwise be unused material (U238). Not only that, the core can run for 40 or even 60 years before needing to be removed and dealt with, which is quite different from the present systems that need to be refuelled every couple of years.
There is also a short video on the topic.
I am a Fusionhead, but still, I think this is interesting technology in the interim. It was also invented by a private research facility.