This is about as simple as a scene can get. Clyde's brother tells him a joke. Clyde laughs. End of scene.
Now, you might watch this and wonder, what is the point? Why is this scene in the movie? Shouldn't it be cut? The scene does nothing to advance the plot. You may argue that it communicates something about the character of Clyde's brother's, but this scene does not tell us anything we did not already observe about him in the two scenes previous.
However, this innocuous diversion does have a narrative purpose. It would have been irresponsible to put it in the film if it did not. Gene Hackman's joke symbolically summarizes the road to damnation that the story's two leads have begun.
Like the little old lady, Bonnie and Clyde first take a little taste of danger and excitement. Then a little more. Then a little more. Pretty soon, they have grown to like the taste. And before they know it, they are hooked. This then leads them to their doom.
Call it a parable. Call it an analogy. Call it thematic foreshadowing. Call it whatever you want. This scene still advances the story by giving the audience new and relevant information. It establishes the theme our story will eventually prove. Like this joke, the best thematic material often works in mysterious ways, and will not be apparent to the audience until the story has ended and they can look back and see the whole picture.