This instruction unconditionally triggers an error, by triggering an exception, stalling or resetting the processor (to be determined). This is the mirror of the NOP instruction but only the six opcode bits are meaningful.
The value of this opcode (all 1s) matches that of unwritten E(E)PROM cells. This way, the core will do nothing if it boots from a blank Flash memory, for example. This opcode should never be used, it's only here to catch accidents/exceptional failure conditions.
The other undefined opcodes would behave the same way, and trigger at least an exception.