Please explain lines from "The Lady of Shalott."
A bow-shot from her bower-eaves,He rode between the barley-sheaves, The sun came dazzling thro' the leaves, 75 And flamed upon the brazen greaves Of bold Sir Lancelot. A red-cross knight for ever kneel'd To a lady in his shield, That sparkled on the yellow field,
The lady in Tennyson's "The Lady of Shalott" is cursed to live in a secluded tower on an island in the river near Camelot. She doesn't know exactly what the curse would be, but she can never leave the tower or look at anyone or anything from Camelot. Rather than looking directly at people and places, she can see them through a mirror that she keeps in her room, and she spends her days weaving a tapestry of the sights she has seen.
One day, like an arrow shot from a bow, the image of the knight Sir Lancelot appears in her mirror. The sun is shining brightly, and it reflects off Lancelot's armor (greaves are the armor plates that cover a knights legs) as if flames of fire. On his shield is the image of a knight "red-cross knight," meaning a knight who has a red cross symbol on his tunic (think of the tunics that Knights Templar wore). This knight is knealing to a lady. They "sparkled on the yellow field" means that the shield is yellow.
The lady is so moved by the image of Lancelot that she can't help abandoning her weaving and going to the window to look down at Camelot to see more than just his reflection. Immediately, the mirror cracks, and she realizes that the curse has begun: " 'The curse is come upon me,' cried the Lady of Shalott."