The following line appears in Canto One of Pope's poem "The Rape of the Lock":

This to disclose is all thy Guardian can.

The line is 113 and lies as the second to last line in stanza 10. Canto One is the first canto of five from the poem.

In Canto One, Pope introduces the Beautiful Belinda. The Sun (a muse in the poem) has been evoked so as to provide the beginning of the day (which the sun historically symbolizes). The people of the household, therefore, are described as going about their typical morning routine.

Readers learn of Belinda and her realtionship with the Sylphs- they are her protectors- through a description of her dreams. Her main protector, Ariel, tells her in her dream that the day will bring about "some dread Event". After awaking from her dream, and the warning, Belinda proceeds to ready herself for the day.

The line in question comes during Ariel's warning to Belinda in her dream. After introducing himself as her protector, Ariel states the following:

E're to the Main this Morning Sun descend.
But Heav'n reveals not what, or how, or where:
Warn'd by thy Sylph, oh Pious Maid beware!
This to disclose is all thy Guardian can.
Beware of all, but most beware of Man!

Therefore, the line's meaning falls in the fact that Ariel is warning Belinda about his knowledge that something bad is going to happen. Although he does not know exactly what, "But Heav'n reveals not what, or how, or where", Ariel knows that he must tell her everything that he does know so as to warn her, "This to disclose is all thy Guardian can."

The direct meaning of the line is Ariel telling Belinda that, at this point, all he can do is tell her about the threat of danger- given the threat is all he knows of.