As well as the allusions to characters from Greek mythology, discussed in previous answers, there are also in this quotation examples of euphemism, repetition, and metaphor.

In this quotation, Romeo is replying to Benvolio, who has told Romeo that a "right fair mark, fair coz, is soonest hit." The word "hit" here is a crude euphemism meaning to have sex with. In other words, Benvolio is telling Romeo that he should have sex with Rosaline as soon as he can. Rosaline is also objectified here as merely a "mark," or a target to be hit with an arrow. The repetition of the word "hit" emphasizes the crude connotations of the word and indicates that Romeo's supposed love for Rosaline is little more than lust.

With this in mind, it seems likely that "Cupid's arrow" might be a euphemistic, phallic symbol alluding to Romeo's penis. This seems especially likely when we consider the frequency of crude and phallic imagery at the beginning of the play. Indeed, not long before, Sampson and Gregory boast about what they will do with the women of the Montague family, with Sampson insisting that, "Me they shall feel while I am able to stand: and 'tis known I am a pretty piece of flesh."

There is also a metaphor in the given quotation when Romeo says that Rosaline is "well armed" with chastity. The implication here is that Rosaline is armed with chastity as a knight might be armed with armor. The armor that Rosaline wears is, to Romeo's frustration, impenetrable. The allusion to armor also implies that for Romeo, conquering Rosaline's chastity is like a battle or a fight to be won.

As the previous Educator has noted, this quote from act 1, scene 1 is an example of an allusion. In literature, an allusion is an indirect reference to a significant person, place or idea. Shakespeare was relying on his audience having a familiarity with both Cupid and Diana to make the allusion have meaning and impact within the play.

It is also worth looking at the comparison between Rosaline and Diana in a little more detail. Diana was the Roman goddess of hunting and chastity. In making this comparison, Romeo is expressing his strong love for Rosaline. The fact that he would compare her to a goddess, a deity, is evidence of that—he has elevated her beyond the realm of a mortal woman. Romeo also believes that, like Diana, Rosaline has deliberately pledged to stay celibate. Of course, we do not know if this is really true, but, whatever the case may be, this statement is effective in highlighting Romeo's frustrations as he tries desperately to woo her.