What is meant in the allusion to Dian’s wit in Romeo and Juliet?

The quote refers to the goddess Diana; she is the Roman version of the Greek goddess Artemis, known as a goddess of chastity, and a patron goddess of virgins, and also of the moon, pregnancy, childbirth and hunting. She is often depicted with a bow and arrow and hunting dogs or a captured deer by her side. It is interesting to see this reference to a moon goddess here, as Romeo meets Juliet soon after...

The quote refers to the goddess Diana; she is the Roman version of the Greek goddess Artemis, known as a goddess of chastity, and a patron goddess of virgins, and also of the moon, pregnancy, childbirth and hunting. She is often depicted with a bow and arrow and hunting dogs or a captured deer by her side. It is interesting to see this reference to a moon goddess here, as Romeo meets Juliet soon after and often speaks of her with sun imagery ("Arise, fair sun and kill the envious moon"), and indeed sun and moon imagery occur throughout the play in Romeo and Juliet's dialogue with and about each other.

When Romeo says "She'll not be hit by Cupid's arrow; she hath Dian's wit, And in strong proof of chastity well-arm'd," he means that Rosalind, his latest love interest, is not likely to compromise her virtue, i.e., have sex with him. "Dian's wit" is another way of saying she is smart enough to not let a young man take advantage of her; loss of virginity before marriage was considered unacceptable for young women, and if sex resulted in pregnancy the young woman's chances of marriage were greatly lessened, not to mention the ruining of her reputation. Romeo does sleep with Juliet soon after they meet, but the two get married in secret first. The moon imagery is also a reference to a woman's monthly cycle, hence why there is an association between these goddesses and pregnancy and childbirth as well.