It simply means that there is more reliance on natural processes to effect things like shutdown and heat removal and less on engineered safeguards. Such passive safety features generally result in higher reliability for safe operation. You don't have to worry about an engineered system failing because processes that rely on the laws of physics will not fail.
The PBMR is one of several Generation IV reactor designs that are being looked at. I am involved in a couple of others, the Westinghouse IRIS design, which is a light water reactor, and the General Atomics MHTGR, which is a bit more exotic. There are some really exotic ones proposed, like the vapor core high temperature reactor, which features first-stage power extraction based on a magnetohydrodynamic generator. Talk about efficiency, that one has it in spades...
The U.S. hasn't built a new commercial nuclear power plant in over 20 years.
And there have been a LOT of technology improvements since the majority of them were built in the '60s and '70s.
The moratorium on new construction has lasted way too long.
It is time for our nation to take advantage of these significant upgrades.