"Dimensional fatigue" (also Inter-Dimensional Fatigue, or "IDF") is a process or force that weakens the boundaries between dimensions. Like "Electromotive force," it's not clear whether IDF is a force in its own right or merely some other process. Dimensional Fatigue can be projected by devices, and occasionally occurs naturally.
IDF research is believed to be incredibly dangerous; most races that use HyperSpace either had a patron such as the Arilou or inherited the technology from elsewhere. The Chenjesu are naturally attuned to HyperSpace, and the Yehat achieved star travel on their own, as did the Humans. A human offshoot culture, the Androsynth, continued IDF research and was destroyed entirely by the Orz, and the Arilou hint that this is not coincidence.
The Arilou may be lying about the danger involved for their own inscrutable purposes; however, it is undeniable that HyperSpace travel was not invented very many times, and that some catastrophe indeed befell the Androsynth.