Just these two words! לא, lo, as you may already know, means "no, not" When used before the imperfect verb form, as in this case, it denotes a permanent command, not just a temporary or conditional command.
תִרְצָח = tav prefix: "you will" (in this case, will NOT! because of LO just mentione)
רצח, rahTSACH = to kill murder, slay
Put it together and we have: "Do not kill"
As always, I welcome questions and comment!