«What is your substance, whereof are you made,
That millions of strange shadows on you tend?».
A more relaxed poet appears to have forgotten his previous doubts about his relationship with the young man, who is still attractive but whose true self is elusive.
What is your substance, whereof are you made,
Ironically, the poet’s lavish and ornate eulogy of the youth — for example, when he compares him to Adonis, a legendary classical beauty — is exactly the kind of affected, stilted, and insincere-sounding poetry which the poet earlier criticized his rivals for indulging in.
The extravagance of the poet’s figures of speech hints at an illusory creature, subtle and complex, perhaps beyond the poet’s powers to describe. Images of shadows, shades, and painting run throughout the sonnet, and the poet’s language employs ambiguous terms — for example, “shadow” may mean silhouette, picture, reflection, symbol, or ghost. Other abstract terms are “substance,” “tires,” and “blessed shape.” Such language indicates the indefinable, cryptic nature of the youth.
English audio from YouTube Channel Socratica
Summary from Cliffsnotes.com