Note to Romeo and Juliet, 3.1.57-59: "I'll be hanged ... man"
Tybalt, seeing Romeo, has just said "here comes my man," meaning, "here comes the man I want to fight." Mercutio pretends to interpret Tybalt's "my man" as "my servant," and says that Romeo is not Tybalt's "man," and does not wear his livery (= kind of uniform for the servants of a household). In the following two lines Mercutio expands on this idea. He says that the only way Romeo could be considered Tybalt's "follower" (= servant, attendant) is if Tybalt went into the "field" (= field of battle, field for duels); in that case, Romeo would follow him and fight him. In such a case, Tybalt could call him "my man," not in the sense of "my servant," but as a brave man who is his opponent.