This passage about Bob Ewells' grudge is significant because, as Atticus points out, Bob Ewell, who is at the bottom of the social stratum of Maycomb, wishes to rise in the estimation of the white community; instead, he is probably thought even less of. Living behind the dump, both literarly and figuratively, Ewell, ironically, with a name that is respected by southerners--Robert E. Lee--seems absurd by comparison.  So, at the trial of Tom Robinson, Ewell essays to make himself seem respectable.  After all, even the lowest desire to have someone else beneath them.  So, when he does not rise in social opinion, Ewell projects his feelings into resentment toward others in the community.

Knowing that the community has nothing but disdain for him fosters his evil intentions of getting even, as well. So, in a sense, this passage foreshadows Ewell's insulting action toward Atticus Finch and his violent act against Atticus's children, both acts which underscore the low opinion of the community.

What Atticus thinks is that Bob Ewell is upset and bitter because the case did not turn out the way he had hoped it would.  Sure, Tom Robinson got convicted and was dead.  But the case did not lead to people thinking any better of Ewell than they ever had.

Atticus explains that he thinks Ewell thought that he would be thanked and respected.  He says Ewell expected he would be something of a hero for getting rid of Tom Robinson.  But instead, people went right back to despising him.

Here is the passage from Chapter 27 that you need:

It might be because he
knows in his heart that very few people in Maycomb really believed his
and Mayella's yarns. He thought he'd be a hero, but all he got for his
pain was... was, okay, we'll convict this Negro but get back to your
dump.