A Krieger wave is a powerful type of energy wave. It had the ability to melt duranium and had an energy signature similar to that of a phaser beam. These waves could be reflected off of transporter beams.
The United Federation of Planets was interested in Dr. Apgar's technology for a potential energy source. However, Dr. Apgar denied having developed them because he had become motivated by profit, and was developing the technology to be used as a weapon instead. Fearing the Federation was suspicious of him, Apgar had intended to kill Commander Will Riker with a Krieger wave, but, instead, the wave reflected off the transporter beam and hit the station's reactor.
During the investigation into the explosion, the USS Enterprise-D accidentally experienced these waves since they had recreated Apgar's converter on its holodeck using his exact design. (TNG: "A Matter of Perspective")