Famous physicists Hiroši Ooguri and Cumrun Vafa proposed a new branch of the Swampland program in their new paper
Non-supersymmetric AdS and the SwamplandIn 2005, Cumrun Vafa coined the term swampland to describe would-be theories or their low-energy effective field theory limits that look consistent according to the rules of effective quantum field theory but that are banned according to the more stringent rules of string theory or quantum gravity (which are ultimately equivalent concepts) i.e. that have no realization within string/M-theory.
The swampland (TRF) is the "larger" but messier realm surrounding the stringy landscape. The swampland shouldn't be confused with the related but inequivalent technical notion of the part of the Internet and media that is critical towards string theory. It's not called a "swampland" but rather a "cesspool" and the technical term for the individuals in the cesspool is "scumbags". The largest and stinkiest two scumbags are known as "Šmoits" but I don't want to overwhelm this blog post with the review of the standard terminology.
The extra constraints imposed by string/M-theory may be interpreted as "general predictions of string/M-theory". They're usually qualitative. Our weak gravity conjecture is the most intensely studied example of such extra constraints. It says that there have to exist particles light enough so that the repulsive electric force between them trumps the gravitational attraction. In this sense, gravity is the weakest force and it has to be.