Is this question asking "What is the nature of the realism in Huck Finn?" Or is it asking "What elements of Huck Finn make make it seem realistic?"
I'm going to assume it's the last question, and if I'm wrong, nverma can let me know.
I'd say that Twain makes the book seem realistic through the following choices:
He writes phonetically, capturing some regional dialect.
His characters are all flawed, as are real people.
The novel's plot is realistic in some ways; it meanders like the river, moving at different paces, just as events in life do.
The descriptions of events are realistic to the character describing them (Huck), as is the voice.
Finally, the nature of Huck's moral dilemmas make the novel seem realistic. He's clearly shown to be a creature of his time.