This is a conditional GoTo instruction. It tests the contents of X, Popping the register stack in the process, then transfers program control (jumps) to the program line LLLL if X was nonzero or True.
This instruction has the alias (alternative name) GoIfT
See also GoTo, GoIfZ, GoIfXEQ GoIfXNE, GoIfXNE, GoIfXGE, GoIfXLT, GoIfXLE, GoIfF, GoIfT