How does mama explain to Cassie about Mr. Simms and Big Ma's handling of the situation in Roll of Thunder, Hear My Cry?

Mama explains that Big Ma was protecting Cassie when she made her apologize.

Cassie is like most young girls.  She is trying to figure out how the world works.  As far as she is concerned, Lillian Jean Simms is just a bratty girl and has no right to tell her what to do.  Lillian Jean  sees it differently.  She considers herself superior because she is white and Cassie is black.  Unfortunately, Lillian Jean has learned to feel this way from her society.  Cassie gets a hard lesson in race and inequality.

The incident occurs because Lillian Jean wants Cassie to get off the sidewalk.  Her brother Jeremy, who is a sort of friend of Stacey’s, tries to tell Lillian Jean that Cassie isn’t doing anything but Lillian Jean responds that Cassie is standing in front of her.  Her father pushes Cassie off the sidewalk, and demands an apology when Big Ma says they are going home. 

“Not ’fore she ’pologizes to my gal, y’all ain’t,” said Mr. Simms.

Big Ma gazed down at me, fear in her eyes, then back at the growing crowd. “She jus’ a child—”

“Tell her, Aunty—”

Big Ma looked at me again, her voice cracking as she spoke. “Go on, child . . . apologize.” (Ch. 5) 

Big Ma knows that there might be trouble if the Simmses do not get what they want.  This is why she is afraid.  She can’t stand up for Cassie then, and she can’t explain things to her.  She just has to get her to apologize so they can get out of there.  It is a terrible thing to see your granddaughter go through, but she has no choice.

Stacey tells Cassie not to blame Big Ma.  He is older, and understands the way the world works better than Cassie does.  Later, at dinner, Cassie complains that Mr. Barnett waited on the whites before the blacks in the store, and when she let him know he banned her.

Mama explains to Cassie that Big Ma was trying to protect her when she forced her to apologize to Lillian Jean.  Mr. Simms acted the way he did because as a white man, he felt superior.

“Because he’s one of those people who has to believe that white people are better than black people to make himself feel big.” (Ch. 6)

Cassie asks why he doesn’t know better.  Mama tells her this is just the way things work.  Although things have changed since the days of slavery, Mama tells her that people like the Simms hold onto the belief that they are superior because “they have little else to hold on to.”  Things are better, but still not equal.