With the Duke and the King aboard the raft, conversation turns to the idea of traveling by day. Up to this point, the two run-aways Huck and Jim have been traveling at night to avoid being seen. Traveling in the day, Jim may be spotted and captured as a run-away slave and Huck may be recognized and discovered to be alive.
The Duke and King understand Jim's situation but are not privvy to Huck's situation. They concoct a plan (a rather dangerous one, it turns out, for Jim) that will allow them to travel by day.
The Duke prints up some posters describing Jim as a run-away slave. The plan is to carry these posters with them so that if they run across anyone who asks about Jim, they can say that Jim is their prisoner. They have captured him and are seeking the reward.
Later, the King uses this poster (with its stated reward) to sell Jim for forty dollars to Mr. Phelps, who plans to recover the entire award state on the poster.