The trilithium torpedo is a superweapon developed by Dr. Tolian Soran. It uses trilithium -- a biproduct of the Federation's matter-antimatter power system -- to cause the rapid collapse of a star. The collapse also produces a powerful subspace shockwave that is destructive to planets, ships, and other objects in the system. It's not clear whether there are limits on what types of stars can be affected or what is the full extent of the damage inflicted on objects in the system, though it is clear that the shockwave will mass-scatter an Earth-sized planet approximately 1 AU from the star.
The only existing examples of the device were either used or destroyed on the launch pad, and all data on its design was lost with the destruction of the Klingon Bird of Prey on which it was carried and Soran's subsequent death. However, other civilizations such as the Dominion are known to use trilithium-based weaponry (albeit to induce a supernova rather than a collapse), so the basic theory behind trilithium weapons appears to be widespread knowledge, even if working examples are much rarer.