llCastRay is a Linden Scripting Language (LSL) function. You may or may not know that SL guns rez and fling prim bullets. Rezzing prims produces lag. Rezzing prims with scripts causes even more lag. llCastRay is a replacement for that process. One may be able to develop weapons with less lag using llCastRay.
Details on the function are in the SL Wiki: llCastRay()

