In Romeo and Juliet, the feud between the Montagues and the Capulets will only be resolved after Romeo and Juliet's love for each other result in their deaths.
Romeo, not yet having laid eyes on Juliet and besotted with Rosaline (who refuses to return his love), plans to attend the Capulet ball uninvited because he knows that Rosaline will be there. He is discussing his situation with his friend Mercutio in Act I, scene iv as they are on their way to the Capulet's ball. Romeo thinks love is very hard and his "soul of lead" (15) indicates how stressed he is; he is convinced that "Under love's heavy burden do I sink" (22). Mercutio recognizes Romeo's feelings for Rosaline as not indicative of real love. It is in his nature to mock love as he finds a far more practical use for it and suggests that Romeo should do the same. Much of what he says has connotations of a sexual nature. Lovelorn Romeo does not think he is in the mood for the festivities that will follow shortly at the ball.
As they are attending a ball, they are carrying masks, a tradition of the time. Mercutio says, "A visor for a visor!" (30). Mercutio is pointing out that in real life, he already effectively wears a mask, not revealing his true self. He is now swapping this "visor" or mask for another—the real mask he will wear to the ball. Both conceal the person behind them.