If we think of the literal meaning of fog, it is something that prevents us from seeing our way. It is difficult to move in fog, difficult to reach our destination.
The symbolic meaning of fog in The Adventures of Huckleberry Finn appears to be similar. It is symbolic of obstacles, things that get in our way and prevent us from achieving our goals.
It is a foggy night that causes Jim and Huck to miss Cairo and freedom. I think the fog symbolizes the confusion Huck feels about helping Jim escape. Huck has been raised in a racist society, and it was against the law to help a slave escape. Huck wrestles with the moral dilemma of helping Jim, but he follows his heart in the end because he sees Jim as a man and not as a slave.
The fog might also symbolize the journey of Huck and Jim as well. Both of them are seeking freedom, but their road to freedom is much like trying to find their way through fog. They are unsure what their next step should be, or what it might lead to.
There are two significant scenes where the fog confuses and misleads Huck and Jim while they are traveling down the river. The first time that the fog surrounds Huck and Jim, they are separated from each other. Huck is in a canoe and loses his sense of direction in the fog. He is unable to paddle back to Jim but eventually reunites with him when the sky clears up. Huck and Jim are again affected by the fog and unintentionally end up sailing past Cairo. In my opinion, I believe that the fog symbolizes Huck's confusion. Huck is struggling to deal with the dilemma of turning Jim into the authorities or helping him gain his freedom. Huck understands and is influenced by society's expectations, but he also has a conscience. Huck's conscience tells him that Jim is a genuine human who is not a piece of property. Huck believes that Jim should be free, but also realizes that he is committing a crime by helping Jim escape. Similar to Huck's decision, the fog is confusing and difficult to maneuver through.