It is vitally important to realise how this important scene in the novel links in with one of the major themes: the coming of age of Huck and his maturing into a character who is able to make decisions often against the values and rules of society. Note how, having been separated from Jim, Huck does what Tom Sawyer would have done and told a massive story about what had happened to try and trick Jim. However, when Jim realises how he has been tricked and communicates to Huck how upset this has made him, Huck shows that he is beginning to think about the feelings of someone else instead of his own fun and amusement.
Not only that, but his action in apologising to a black slave shows Huck's first victory over the values of his society. For a white boy in a world where whites were superior to blacks to apologise to a runaway slave was unthinkable, therefore indicating the way that Huck is learning to listen to his moral conscience more than the dictates of society:
It was fifteen minutes before I could work myself up to go and humble myself to a nigger; but I done it, and I warn't every sorry for it afterwards, neither.
Symbolically, then, his actions have much more importance than they seem to do at first glance.
The fog might symbolize Jim's and Huck's blindness to the fact that Jim was already a free man. They were striving and taking so many pains to conceal Jim when there was no need at all, but at the same time, there was no way they could have learned it, the only option left for them was to "try to find their way in the fog".
Moreover, in their journey they learn many valuable lessons, like the fact that they are so impoartant to each other. At that time, friendship between blacks and whites was frowned upon. Thus, the fog also represents the difficulties they met before realizing that frienship and mutual help can defy and break the impositions placed by society.
Their being able to finally find each other in the midst of the fog might foreshadow the end, when everything turns out to be alright.