An allusion is a reference to a person, place, incident, situation or thing whether it is from the past or current to enhance or emphasise a point or statement. The reference may be direct or indirect and it is up to the audience to ascertain its effectiveness within context.
In Scene1 of Act 2, Mercutio are discussing the lovelorn Romeo when Mercutio says the following:
Speak to my gossip Venus one fair word,
One nick-name for her purblind son and heir,
Young Adam Cupid, he that shot so trim,
When King Cophetua loved the beggar-maid!
Mercutio is here alluding to the much revered Roman goddess, Venus, who is regarded as the epitome of love, sex, beauty, charm, fertility, sensuousness and feminine seduction within the community of immortal gods. He furthermore also refers to Adam, who had been a much-admired archer in Shakespeare's time for his skill, and Cupid, believed to be the son Of Venus.
It was believed that a shot from Cupid's arrow would weaken even the most powerful and resolute and fill them with lust and desire and an overpowering amorous need that had to be fulfilled.
A further allusion is to King Copethua. Legend has it that the king lacked any sexual attraction to women. One day, he noticed a beggar woman in the street whilst looking out of his window, and was instantly attracted to her. He went outside, offering gold coins to all the beggars and, when she approached him, told her that she would be his wife. He later married her and she became queen.
The point that Mercutio is making is that Romeo is so overwhelmed by love that he is blind to everything and everyone.
He heareth not, he stirreth not, he moveth not;
The ape is dead ...
Romeo does not even respond to even a mention of the names of the characters Mercutio alludes to. His only recourse is to draw Romeo closer by conjuring (casting a spell on) him, using references to Rosaline as part of the conjuration or spell.