Huck and Jim get separated in the fog for a while. When they reunite, Huck finds Jim and Jim is asleep. As Jim wakes up, Huck is just sitting there. Then Huck plays it off as if he has been there the whole time and Jim dreamed the fact that they were separated.
The separation certainly worried Jim. This demonstrates that Jim is incredibly gullible and immature. If a kid can pull this lie on him and get away with it, this shows us that his intelligence level is pretty low. On Huck's part, it might show one of two things. First, if Huck did it out of spite or meanness, that says he's pretty immature too. But, if Huck did it to help make Jim's fears of being separated go away, then that is a pretty kind and mature thing to do.
If Huck did it to be mean, it shows that he has little concern for humanity and does not value Jim as a human being with the right to be treated fairly. Jim's reaction demonstrates great care and concern for Huck, thus, great favor toward humanity.