Kim has not just one, but two amulets. The first one is a little bag put together by the woman who looks after Kim when his parents die. It has three documents, all of which belonged to Kimball O'Hara, Sr. The first is Kim, Sr.'s "ne varietur" (1.2) as he calls it. Ne varietur is a Latin legal term meaning, "Not to be changed." Masons would put this phrase underneath their handwritten signatures to verify the handwriting of individual Masonic brothers. So Kim, Sr.'s ne varietur is a document proving that he is a Mason. (The Freemasons are a mysterious fraternal organization that claim to go back to the time of Solomon.)
The second document in Kim's first amulet is his father's clearance-certificate, which provides legal proof that Kimball O'Hara, Sr.'s debts have all been paid and that his estate is settled. This is great—Kim doesn't owe anyone money on behalf of his father. Last but not least is Kim's birth certificate, giving the legal names of his mother, Annie Shott, and his father. When Kim meets up with Reverend Bennett and Father Victor, these documents prove who Kim is, even though Kim doesn't really seem to know himself.
Kim's second amulet comes from the Babu. Like Kim himself, the look of this amulet is deceptive: It looks like a cheap silver sign that he is part of the extinct Sat Bhai cult, but really, this amulet proves that Kim is part of the Secret Service. And this amulet comes in handy right away, as Kim is on his first train trip after leaving school—his knowledge of the amulet's meaning allows him to help Agent E.23 to escape his pursuers.
Both of Kim's amulets tell us something about who he is: the first one proves his British origins, even though he is dressed as an Indian child and speaks Urdu fluently. The second one demonstrates that Kim is and always will be the Son of the Charm—a member of this vast, secret network of government spies who keep British India under watch. In both cases, Kim can only be identified by this one thing he wears around his neck. After all, he changes everything else about himself so fast and so often. The amulets are the only constants about Kim.