Explain what Hamlet is saying in this verse with Gertrude: Look here, upon this picture, and on this, The counterfeit presentment of two brothers. See, what a grace was seated on this brow; Hyperion's curls; the front of Jove himself; An eye like Mars, to threaten and command; A station like the herald Mercury New-lighted on a heaven-kissing hill; A combination and a form indeed, Where every god did seem to set his seal, To give the world assurance of a man: This was your husband. Look you now, what follows: Here is your husband; like a mildew'd ear, Blasting his wholesome brother. Have you eyes? Could you on this fair mountain leave to feed, And batten on this moor? Ha! have you eyes? You cannot call it love; for at your age The hey-day in the blood is tame, it's humble, And waits upon the judgment: and what judgment Would step from this to this? (3.4.55)

Hamlet, in a frenzy, has approached his mother when she is alone in a private chamber. (He does not know Polonius is listening behind an arras, or tapestry.) He wants to confront her about her marriage to Claudius, of which he highly disapproves. To do so, he presents her with a miniature painting of his father, her late husband, and one of Claudius, placing them side by side. This is what he means when he says he is showing her a "counterfeit presentment" of the two brothers.
Hamlet points out how much more noble and handsome his father, the late King Hamlet, was compared to Claudius. Hamlet compares his father to gods such as Jove (Zeus) and Mars. He then has Gertrude look at the picture of Claudius, who Hamlet calls a "mildewed ear" in comparison, probably thinking of Claudius pouring poison into his father's ear, but also pointing out how much lesser of a man his uncle is. He says his father is a mountain and Claudius a lowly plain, a "moor."
Hamlet also says to Gertrude that it could not have been the blindness of romantic love that led her to Claudius because she is too old for that. She must have married using her reason, and he then asks what woman of "judgment" (reason) would pick a low-life man like Claudius?
We should keep in mind that Hamlet is mourning the loss of his father and has a highly idealized view of him. Nevertheless, his words—and showing the pictures side by side—does hit home. Gertrude responds with anguish, suggesting she realizes how much she has "lowered" herself in marrying Claudius and all but admitting it was a marriage of convenience.

Hamlet is furious with his mother for marrying the man who killed his father, Gertrude's late husband, King Hamlet. Gertrude is unaware of Claudius's role in King Hamlet's death, but even so, Hamlet resents her for remarrying so soon, while he was still in deep mourning for his father.
In the above excerpt, all of Hamlet's bitterness, bile, and rage towards his mother come to a boil as he subjects Gertrude to a vicious, unhinged rant. During this outburst, Hamlet draws an unfavorable comparison between his late father and Claudius. He compares King Hamlet to a Greek god, with his curly hair and noble forehead. Then there's Claudius: he's like a mildewed ear of corn infecting the one next to it. The difference between King Hamlet and Claudius is night and day, and Hamlet simply cannot believe that Gertrude can't see this. Hamlet reckons that some evil demon must have blinded her when she chose to marry Claudius. How else can one account for what, to Hamlet, is such an inexplicable choice?

What devil was ’t
That thus hath cozened you at hoodman-blind? (act III, scene iv)

In this speech, Hamlet is confronting Gertrude, his mother, with the difference between her first husband, King Hamlet, and her second, Claudius, King Hamlet's brother. Hamlet describes the stark differences between the two in describing the first, King Hamlet, in terms that allude to the Roman gods. He has "the front of Jove himself" and "an eye like Mars, to threaten and command." Allusions to the king of the gods and to the god of war suggest that King Hamlet was well fit to be a king, having the attributes of these two gods particularly. Indeed, this man is the epitome of what a man should be: "to give the world assurance of a man / This was your husband."
By contrast, Claudius does not hold up to comparison. Unlike his brother, Hamlet is saying, he is "like a mildewed ear." The imagery in this statement is vivid and deliberately repulsive. Hamlet demands of his mother, "Have you eyes?" He is saying, can you really be seeing this man as I see him, and yet imagining that he compares well to your previous husband? Hamlet pursues the question, suggesting that he cannot even understand it in terms of lust, because "at your age / the heyday in the blood is tame." His mother is too old, Hamlet thinks, for lust to have blinded her to Claudius's faults, and yet he cannot understand how her "judgement" could have had her "step from this [King Hamlet] to this [Claudius]." He is saying he cannot understand why she would stoop so low from such a commendable husband to such a "mildew'd" one.