need explanation of these linesAnd sometimes thro' the mirror blue 60 The knights come riding two and two: She hath no loyal knight and true,...

The Lady of Shalott can only view life through a mirror
due to a curse placed on her.  She is not allowed to look directly on any scene outside
her window.  So, these lines are saying that sometimes when she looked at the  mirror,
she would see two knights heading towards Camelot two by two.  They would go down the
road in sets of two, beside one another. Having a knight defend your honor and show his
love was a wonderfully romantic idea during that time.  Not having ever met anyone in
person, she did not have a knight to love her and fight for her.  However, and remember
she wove what she saw into her tapestry, she could imagine what it would be like and
delight in it.