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.