in the play Romeo and Juliet what is a foil? how does Mercutio act as a foil for Romeo?

A character that reflects (like shiny foil) the attributes of another character is a foil character. One character brings out the traits of another specific character. Mercutio is outlaish and fiercely protective of Romeo. Mercutio preaches loyalty and comradery above all traits. Romeo is more insular, fretting about love or lost love. It isn't until Mercutio is killed that we see his (Mercutio's) traits surface within Romeo. Romeo temporarily changes from a heartsick adolescent into a loyal friend seeking revenge.