The converter was essentially a set of mirrors and dicosilium reflective coils which converted Krieger waves into existence by focusing energy pulses from a Lambda field generator. On the surface of Tanuga IV, such a generator was aimed at the research station. It was physically located on the planet itself, since a minimum distance of 5,000 kilometers was required for the lambda field to collimate.
Although the wave converter was originally intended to provide a new energy source, Apgar had more sinister motives, and he developed the converter to be used as a weapon, which would have been more profitable to him. To this end, he aimed to build larger reflective coils, which required more dicosilium. When, after refusing to provide the dicosilium to Apgar, Enterprise Executive Officer Commander William T. Riker signaled his readiness to return to the Enterprise there and then, Apgar tried to assassinate him using the Krieger wave converter, but instead only succeeded in killing himself when the Krieger waves deflected off the transporter's annular confinement beam directly into the converter itself, as a direct result of which his station exploded. (TNG: "A Matter of Perspective")
The Krieger wave converter was named for David Krieger, a science consultant to Star Trek: The Next Generation, who even worked out a pseudo-scientific explanation for what Krieger waves actually did. But ultimately, the converter was relegated to the menial status of a "MacGuffin" story element.