The answer to this question can be found in Chapter Two of this coming-of-age classic. Having jumped out of his window to be with Tom, Huck passes Jim, and the two boys plan a trick on him. Jim has fallen asleep and so Tom takes his hat and hangs it from a branch of the tree he is sitting against. Note how Jim interprets this "sign":
Afterwards Jim said the witches bewitched him and put him in a trance, and rode him all over the State, and then set him under the trees again, and hung his hat on a limb to show it.
Jim became "monstrous proud" of the way that he had been used by witches, and it is clear that he gained a certain popularity because of it, as he was regarded as something of a "wonder." The irony of this lies in the fact that of course it is merely Jim's overactive imagination and belief in superstition that leads him to conclude that a silly, childish prank was the activity of witches. It does however serve to introduce the important theme of superstition into the novel, which, as is made clear, both Jim and Huck are subject to, to varying degrees.