Giant, the

Giants appear as rhetorical figures in a handful of Stevens' poems, including "Repetitions of A Young Captain, "Notes Toward a Supreme Fiction," "The Owl in the Sarcophagus," "A Primitive Like an Orb," and "Owl's Clover: A Duck for Dinner." The Giant, however, is the title character in Stevens' "The Plot against the Giant," in which the otherwise fearsome creature is reduced to "le pauvre" (the poor thing) by the machinations of three, presumably teenage, certainly pubescent, girls. This character, though only a single example of Stevens' use of the figure, nevertheless exhibits all the general features of Stevens' giants, and can thus stand as a representative example.
What is a Giant?
The giant is a mythological monster: a humanoid creature of massive size, generally antagonistic either to the Gods, to humanity, or both. The word "giant" derives form the Greek "gigantes," but legends of giant peoples are common across Indo-European and Middle Eastern mythologies. Some famous giants include: Gog and Magog, Goliath, Orestes (of Greek mythology), the frost giants of Norse legend, and, of course, the giant in the fairy tale Jack and the Beanstalk ("Giant (mythology)" Wikipedia). 
What is the Giant Doing in Stevens' Poem?
Losing, mostly. As noted, the poem takes readers' through a chronicle of the giant's defeat, related in a highly suggestive, sexualized diction. Intriguingly, Stevens nickname at Harvard was "Giant" (due to his imposing size), and he refers to himself in this way a number of times in his letters as well as in other poems, including "Large Red Man Reading" and, arguably, "Bantams in Pine Woods." Generally, these self-portraits are self-deprecating, and so it is easy to read "The Plot" as a humorous, whimsical satire of Stevens' own youthful experience of sexual desire. As he himself put it in an interview with the New York Times : "In my youth, I liked girls. But let's not stress that, I have a wife."
However, poet William Siglar makes a convincing case that "The Plot"'s overtly sexual word-play operates on a number of levels, providing "a correlative for the aesthetic response" to nature's sensuousness, as well as, plausibly, a meditation on the difficult relationship between the accoutrements of language as poetry, symbolized in the poem by the "civilist odors," the "cloths besprinkled with colors," and, most significantly, the "heavenly labials" which, by turns, check, abash, and undo the giant (Siglar). Given that Stevens, in his letters, links the giant as a rhetorical trope in his poems explicitly with "the hero, [or major man"] , it is also possible to see the undoing of the giant in this poem, along with Stevens' other playful deployments of the giant, as a sign of Stevens' skepticism as to the possibility of developing a universalized, representative figure of humanity. Thus the giant, and his defeat, comes to echo the disintegration of the real amid the seductive particularities of a linguistically structured reality.