In chapter 20, Atticus makes his closing remarks by proving Tom Robinson's innocence, providing insight into Mayella's motivation to fabricate her story, and encouraging the jury to conduct a fair assessment of the case. After briefly discussing the prosecution's lack of evidence and mentioning the conflicting testimonies of their witnesses, Atticus elaborates on Mayella Ewell's motivation to falsely accuse Tom Robinson of assaulting and raping her. Atticus begins by stating that he has nothing but pity for the state's chief witness but accuses her of putting Tom Robinson's life at stake in an effort to get rid of her own guilt.

Atticus goes on to state that Mayella's guilt was her primary motivation for falsely accusing Tom Robinson. Although Mayella did not commit a specific crime, she did break the "rigid and time-honored code of our society" of tempting a black man. In Maycomb's segregated, prejudiced society, it is considered taboo for a white woman to have relations with a black man. By kissing Tom Robinson and making advances toward him, Mayella broke her society's unwritten code and committed an "unspeakable" act in the eyes of her prejudiced community members. In an attempt to remove the evidence of her offense, Mayella falsely accused Tom Robinson of assaulting and raping her. According to Atticus, Tom Robinson was a daily reminder of her offense, and Mayella was confident that her white skin would erase her guilt in a court of law.

Chapter 20 of To Kill a Mockingbird is where Atticus gives his closing arguments in the rape case against Tom Robinson. Since there is no clear evidence to prove that Tom Robinson raped Mayella Ewell, Atticus does his best to discredit his white accusers. The whole case is ironic because the true victim is on trial for a crime he didn't commit; so, Atticus sheds light on the person who is the true offender in the case--Mayella Ewell. Atticus specifically says that Mayella didn't do anything illegal, per se; but she did do something socially wrong according to the current mindframe of the South in 1935. Atticus explains as follows:

"What was the evidence of her offense? Tom Robinson, a human being. She must put Tom Robinson away from her. Tom Robinson was her daily reminder of what she did. What did she do? She tempted a Negro" (203).

No socially respectable white woman of the time period would even consider tempting, or throwing herself at, a black man. Mayella is low-class, lonely, and not respectable. That must be why she thought she could get away with setting Tom up for her to kiss him. When her father caught her, they had to blame it on the black man in order to save Mayella's reputation, no matter how low it was.

In Chapter 21, we hear of the decision of the jury. As everyone is waiting, there is a feeling of uncertainty. The children, particularly Jem, believe that the verdict will be innocent, but the adults know that Tom Robinson will most likely be found guilty. 

Reverend Sykes tells the optimistic Jem these words: 

In light of this point, the wrong that Mayella committed against Tom Robinson is to blame him for a crime he certainly did not commit. She was covering up her shame, because she knew that she could get away with it, even if all evidence was against her, simply because she was a white woman and Tom Robinson was a black man. She knew the people, the town, and the outcome. So she worked the system to her favor. 

In Chapter 20 Atticus gives his rationale more clearly. 

Her crime, as Atticus says in Chapter 20, is that she "was white, and she tempted a Negro."

Atticus, in his summing up, says that Mayella Ewell has "put a man's life at stake" (i.e., Tom Robinson's), "in an effort to get rid of her own guilt."

He then clarifies that Mayella was not guilty of a crime, but she was guilty of breaking a rigid social code:

"... a code so severe that whoever breaks it is hounded from our midst as unfit to live with. ... She was white, and she tempted a Negro.  She did something that in our society is unspeakable: she kissed a black man.  Not an old Uncle, but a strong young Negro man.  No code mattered to her before she broke it, but it came crashing down on her afterwards."

Notice that Atticus does not say this social code is right, merely that Mayella Ewell knew she was "guilty" of breaking it.  He goes on to say that, in order to escape the severe consequences that would come her way if she admitted what she'd done (social ostracism, and probably more savage beatings from her father), Mayella desperately tried to blame Tom Robinson by accusing him of rape.  

Atticus has great sympathy for Mayella.  He knows she leads a lonely life, and that Tom Robinson was the only person in her world who was kind to her.  He understands why she was tempted.  However, he cannot spare her feelings by keeping her secret.  He must bring her secret out in order to save Tom Robinson's life.