In this scene, Romeo, Mercutio and Benvolio have just fled the party in haste, for fear of being caught. At the last minute, Romeo turns and runs back toward the palace, leaping the wall into Juliet's garden.
Neither Benvolio nor Mercutio know why Romeo went back inside, but both likely presume it was to fight. Mercutio here thinks he is being very clever, to entice Romeo back out with the idea of getting his mind off fighting, and onto a girl. The irony is two-fold. First of all, he chooses the wrong girl (Rosaline instead of Juliet). Secondly, by crassly naming off different body parts (partly done in Mercutio's crass humor), he is attempting to appeal to the only side of Romeo that would take his mind off fighting. If Romeo could actually hear Mercutio at this point, such an appeal would only encourage him further in the direction he is headed.
There is no mention of fighting in this scene other than by Tybalt which is not spoken in Romeo's presence. However, keep in mind that Romeo has been depressed that his love, Rosaline, has taken a vow of chastity. Therefore, his cousin, Benvoliohas offered to show Romeo all of the other women Verona has to offer so he will think Rosaline is a crow not a swan. When the opportunity to attend the Capulet party arises and Rosaline's name is on the guest list, Benvolio plans on having Romeo attend. En route to the party Mercutio tries to lift Romeo's spirits even though Romeo insists he has a soul of lead. Once at the party, we can assume Romeo sees Rosaline since her name was on the guest list. Yet we know he meets and falls for Juliet. After the party, Romeo is so enamoured with Juliet and so in shock as he has just found out she is a Capulet, that he wants to be alone with his thoughts (similar to his desire to be alone to ponder Rosaline's chastity vow instead of being his love). He quickly climbs a wall without considering the owner of the property in order to escape his friends who he hears approaching according to the lines and stage notes. As his friends approach they yell his name but do not get a response so Mercutio, beingthe attention seeking clown he is, conjures Romeo to appear by using the "magic" of Rosaline's body. This is ironic because at this point Romeo has met Juliet and in turn seen Rosaline as a "crow" which would be the last thing that would cause him to appear.