In this line, Mercutio uses a pun or double entendre, which is a word or phrase that contains two or more meanings. Ever the wit and wordsmith, Mercutio, though he is dying, can't resist indulging in a last play on words. His speech begins as follows, responding to Romeo's wishful hope that his wound is not every deep:
No, ’tis not so deep as a well nor so wide as a church-door, but ’tis enough, ’twill serve. Ask for me tomorrow, and you shall find me a grave man. I am peppered, I warrant, for this world.
Mercutio is saying that, tomorrow, he will be in his grave, but he also will be grave or solemn in demeanor, because he will be dead. As Mercutio knows, Romeo shares his special appreciation for words and will understand the bitter irony of his pun. Mercutio's words also foreshadow Romeo's fate. Both promising young men will be dead as the result of a pointless feud. The grief and frustration we might feel at Mercutio's senseless death foreshadows how we will feel about Romeo's.
It is worth mentioning that "peppered," in its archaic meaning, is also a pun—Mercutio is saying, as any of us might, that he is peppered with wounds but also that he has been inflicted with severe suffering (the archaic meaning) and that he is spiced (peppered) or prepared for burial.
Mercutio is a vibrant, compelling, lively character, and he stays true to form even as he is dying, always the witty realist.