Hamlet watches the army of the Norweigan, Fortinbras, and asks a Captain where they are going. The Captain tells him that the Norweigans march to "gain a little patch of ground" which is barely worth five ducats. The army of men are about to pointlessly sacrifice their lives for a seemingly worthless gain.
Starting from this point, Hamlet's soliloquy considers the way that everything he experiences ("all occasions") seem to point toward the revenge regicide--dulled by inaction, "dull revenge"--that he has thus far been unable to commit against Claudius.
Hamlet ruminates on the nature of humankind's God-given reason and concludes bitterly that he does not know why he remains alive saying that "this thing's to do" but still not doing it, a remark that makes explicit the dichotomy between thought and action that many critics have found in the play as a whole.
As he did in his earlier soliloquy ("Oh what a rogue and peasant slave..."), Hamlet considers that, unlike these soldiers, he has a "great argument" (compelling reason) to carry out his deed of vengeance, but, unlike these men, he seems unable to do it.
Hamlet's conclusion is that, from this moment forth, "my thoughts be bloody, or be nothing worth" though we might detect a wry irony of Shakespeare's in the line: Hamlet, who is supposed to be doing has just resolved to think bloodily, rather than act bloodily.