The following lines appear in Canto Two, lines 105-106, of Pope's poem "The Rape of the Lock."
Canto Two delivers the main conflict of the poem. Belinda is a beautiful woman with locks of hair that many desire. While out on a boat, Belinda's locks are looked upon by the Baron. The Baron is so obsessed with Belinda's locks that he had previously built an alter so that he could ask for success in obtaining the locks.
At the end of the canto, the following lines appear:
Whether the Nymph shall break Diana's law,
Or some frail China jar receive a flaw,
The lines refer to the charge the nymphs have in regards to Belinda. In Canto One, the lines depict the fact that nymphs have been charged with the position to watch out for and protect Belinda based upon the fact that Ariel has had a premonition that harm will come to Belinda.
The lines simply mean that, regardless of the laws set, the nymphs will do what ever needed to protect Belinda so that the "frail China jar" (Belinda) will not be damaged.