Everything we know in Tennyson's poem "The Lady of Shalott" we learn from the narrator, who tells the story from a third-person omniscient perspective. The narrator informs us early on that a magical being referred to as the Lady of Shallott is imprisoned (by curse) on an island near Camelot. She is trapped in a small castle there in Shalott:

No time hath she to sport and play:

A charmed web she weaves alway.

A curse is on her, if she stay

Her weaving, either night or day,

To look down to Camelot.

She knows not what the curse may be;

Therefore she weaveth steadily,

Therefore no other care hath she,

The Lady of Shalott.

The Lady is here said to weave a magic web. She observes the people and things that pass below through a magic mirror (weaving them into the web), but she cannot look upon them directly. If she were to look outside the window, the curse would take effect (though the poem is vague about the curse itself—its origin and cause, for example). On one occasion, she does look out because she spies Sir Lancelot and this causes her mirror to break and her magic web to fly out of the window. Knowing the curse doomed her, she dies in the final part of the poem.

"The Lady of Shalott" is a poem by Tennyson narrated in the third person. The poem opens by describing an idyllic scene with a river surrounded by flowers and trees running from Shalott to Camelot. On an island in the river there is a tower inhabited by an nameless "lady" who is is elegantly dressed and whose singing can be heard by people passing by the tower. 

In the first stanza of Part II, the narrator tells us that the Lady of Shalott is cursed and must constantly weave and never stop weaving to look down upon Camelot. She can watch the world as reflected in a mirror but looking directly at the outside world will cause the curse to kill her. Thus the curse keeps the Lady imprisoned in her tower in Shalott. We deduce this from information given us by the narrator.