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...

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.