What does Mercutio mean when he talks about conjuring?

When Mercutio mentions conjuring, he is referencing the use of magic or trickery to create an illusion or manipulate events mysteriously. He is using this metaphor to express his belief that Romeo is acting foolishly and pretending to be deeply in love with Rosaline when he is not. Thus Mercutio accuses him of being so much under love's spell that he sees Rosaline as more than she is, conjuring up an illusion of her beauty and worth.