In "The Last Ride Together" by Robert Browning, explain the stanza, "Hush! if you saw some western cloud. . . moment on my breast."

The complete stanza reads as follows:

Hush! if you saw some western cloudAll billowy-bosom'd, over-bow'dBy many benedictions—sun'sAnd moon's and evening-star's at once—And so, you, looking and loving best,Conscious grew, your passion drewCloud, sunset, moonrise, star-shine too,Down on you, near and yet more near,Till flesh must fade for heaven was here!—Thus leant she and linger'd—joy and fear!Thus lay she a moment on my breast.

This stanza probably most profoundly expresses the speaker's overwhelming bliss when the mistress, who has rejected him, takes a moment to rest her head on his chest. The feeling of joy he experiences at this point is so overwhelming that he does not immediately share it with the reader. Instead, at the beginning of the stanza, he asks for silence and then leads the reader to the climax of what he perceives as his greatest pleasure.
The speaker gradually builds up to the climax by addressing the reader directly. By using images from nature, more specifically, heavenly visions of clouds that are all in full swell, he draws the reader in by referencing the reader's own experience and joy at witnessing such a heavenly spectacle. He refers to this wondrous scene as being laden with the most beautiful of nature's wonders: suns, moons, and stars. The reader is reminded that seeing these all at once is truly a gift.
The reader is involved further when the speaker assumes that the reader would look and love this sight the most and that, while being overwhelmed, would also have felt his passion drawing these heavenly bodies ever closer to him until the physical reality disappeared to become a wholly spiritual experience. The experience became so overwhelming that it could not be experienced in a physical sense; the flesh had to fade and was replaced by heavenly ecstasy because the experience was ethereal and spiritually uplifting.The exclamation mark at the end of the line emphasizes the speaker's rapture.
The last two lines of the stanza explain the reason for the speaker's overwhelming pleasure. His mistress had, just as these images did, leant over him and stayed a moment. The break in the line indicates how much he was overcome by her simple act, to the point that he experienced both joy and fear simultaneously. He feels joy for his immediate pleasure and fear because he knows the moment would not last or he might not have known how he would respond.
The repetition of "thus" in the final line of the stanza emphasizes how important it was for the speaker that his mistress took time to lay, probably her cheek, on his breast.
The importance of this moment should be understood within the context of the entire poem. It is a dramatic monologue in which the speaker remembers the rejection of a lost love. The speaker recalls their last ride together and the memories he has of that time. Throughout his monologue, the speaker comments about success and failure and the reward for hard work and commitment, which, more often than not, amounts to very little or even nothing.
The speaker does not feel like a failure, though. Having spent time with his mistress more than makes up for everything, even though she rejected him. It is enough for him that she actually went on this one last ride with him, as it will be a memory which he will treasure for all eternity.