There are a number of ways to interpret this poem. Looked at literally, as mizzwillie does in her post above, the poem resonates with ideas of loneliness, isolation and eventually a yearning for life.
However, if we read the poem more figuratively (metaphorically), we might see "The Lady of Shalott" as a poem about death.
The Lady in the poem is repeatedly associated with death, from Part I all the way to Part IV.
The first connection to death comes early in the poem.
The reaper, reaping late and early,Hears her ever chanting cheerly,Like an angel, singing clearly
The "web" that the Lady is said to be weaving might be taken to be a shroud (used to wrap a corpse). The Lady's tendency to reflect and look back (or backwards through a mirror) also aligns nicely with the notion that she is emblematic of death, seeing life only in retrospect.
But in her web she still delightsTo weave the mirror's magic sights,For often thro' the silent nightsA funeral, with plumes and lightsAnd music, came from Camelot
A pearl garland winds her head:She leaneth on a velvet bed,Full royally apparelled
The poem, The Lady of Shalott, has several meanings or themes. The lady is in isolation, locked in a tower and under a curse though we are never told why. She lives her life passively through a mirror which is to say that she really doesn't live in the real world because the reflection of the mirror is her world. Anyone who lives passively eventually wants to join the real world as the lady does when she hears Lancelot sing. Now, she joins the real world when she looks at Camelot because of the singing and in essence sets the curse in motion. For the first time, she can now choose what to do with the rest of her life which is to float down the river in a boat and see the world along its banks. She dies, but before her death, she lives in the reality of life instead of the distance of a mirror. In death, she is without a name and in the end is treated as an anonymous lady from a place called Shalott.