Huck and Jim are both superstitious. Huck's superstitions are pretty common ones - such as throwing salt over your shoulder to ward off bad luck if you spill some salt. He also believes it is bad luck to kill a spider or touch a snake. Although he is superstitious himself, Huck makes fun of Jim for his superstitions because he thinks Jim's superstitions are silly.
Jim's superstitions, Huck believes, are negro superstitions and therefore sillier than white superstitions. Jim believes, for instance, that a hairball can tell the future. He also believes that if two chickens fly into a yard at the same time, it means rain, or if you count the things you are going to prepare for dinner, that brings bad luck. If one shakes a tablecloth after sundown, that is bad luck. If a beekeper dies and the bees are not sold before the next morning, that is bad luck. Lots of hair on one's arms or legs means riches.
Ironically, many of Jim's superstitions in the novel turn out to be true. Huck slowly comes to appreciate Jim's knowledge and experience and when Jim's superstitions come true, Huck begins to doubt his own beliefs and questions the traditions with which he has been raised by the Widow Douglas and others. What society has taught Huck turns out not to be as true as the folk wisdom Jim has learned through experience.
Myth and superstition are one of the motifs of this novel. Read about it here on enotes.