Trilithium is a compound that can halt the nuclear fusion of a star; Tolian Soran used it to destroy two stars in his bid to re-enter the Nexus in 2371.
Dr. Tolian Soran developed a trilithium-based weapon able to collapse a star, and the Duras sisters supported his work in hope of acquiring it for themselves. Soran tested it on the star near the Amargosa Observatory, destroying it and causing the deaths of many people.
His real goal was to alter the course of the energy ribbon known as the Nexus so it would pass over Veridian III, where he could rejoin it. That required destroying the Veridian star as well, despite the populated world in that system. Picard and Kirk stopped him on Veridian III before the launch succeeded.