In Tennyson's "The Lady of Shalott," what is the poet saying here:
By the margin, willow veil'd,
Slide the heavy barges trail'd
By slow horses; and unhail'd
The shallop flitteth silken-sail'd
Skimming down to Camelot:
But who hath seen her wave her hand?
Or at the casement seen her stand?
Or is she known in all the land,
The Lady of Shalott?
In the early stanzas of Tennyson's "The Lady of Shalott," the setting is established, as is the contrast between Shalott and Camelot. Specifically, though, the stanza you quote establishes that much traffic passes by Shalott on the way to Camelot, but no one takes notice of the Lady.
Barge traffic passes by, the stanza establishes, and the barges are heavy, big. The idea is that many people pass by everyday, but no one sees the Lady. The narrator rhetorically asks whether or not anyone is even aware of her existence.
A slight suggestion may exist in the lines that the people that pass by are negligent in not paying attention, just as in the close of the poem the suggestion that Lancelot has been negligent may also exist. For the most part, however, the poem establishes that the Lady is not allowed to look out the window (the casement) and wave. She is cursed. She is isolated--that's the point. She is an artist totally isolated from that which she is supposed to be depicting--reality. The stanza you quote contributes to the establishment of that isolation.
Of course, it also raises reader sympathy for the Lady.