Explain why the men have come to the jail.

This particular scene is short, but very interesting to
explore.  In general, the men have come to get Tom Robinson. They do not want him to
stand trial, and they are upset that Atticus Finch has decided to defend him.  Instead,
they want to wreak their own sense of mob justice, inflamed by racial
hatred.

The men want to assert their white supremacy. 
Particularly, these men are poor white trash.  The comment is made that these men are
the most threatened by kind or fair treatment of blacks because it endangers their
slightly higher status on the social ladder.

Beyond this,
the theme of racism is illuminated through the eyes of the Finch children.  Atticus,
anticipating this possibility, has taken up post outside the jail.  His children,
without permission, come to spy upon the scene, revealing themselves out of fear for
their father.

Luckily, the presence of the children, their
innocent outlook on the mob (particularly Scout's recognition of one of her friend's
fathers), and the calming presence of Atticus diffuse the situation and prevent a
violent riot.