The testimony of Mayella Ewell can be found right at the start of Chapter 18. The basic idea is that she says that Tom Robinson raped her by force.
She says that she was hanging out on her porch when he walked by. She called to him and told him to come break up a "chiffarobe" for her -- she would give him a nickel if he did.
When she went into the house to get the nickel, he grabbed her. They fought. He threw her to the floor and "took advantage" of her.
In addition, Mayella's testimony draws the reader to seriously question her credibility as a witness.
First, Mayella approaches her testimony afraid of Atticus, a most gentle attorney who never raises his voice, but searches for truth and will do so in a way that challenges a witness, like he did with Bob Ewell.
Atticus uses her testimony to build the argument that she is a victim of her circumstances, and that there is potential that she has been neglected and abused. This is something that Tom Robinson has no responsibility for. She has no friends, and is therefore lonely. She performs the functions of a parent for 7 other children and has been robbed of an education. She is a likely candidate to give lies in her testimony.
Mayella Violet Ewell is called to testify in the trial of Tom Robinson in Chapter Eighteen of To Kill a Mockingbird. Mayella takes the stand and asserts that she was sitting on the front porch of her family's home when Tom Robinson passed by. Mayella claims that she had asked Tom to "bust up" the old chiffarobe for her in exchange for a nickel; when Mayella went into the house to retrieve the nickel, she states that Tom followed her, grabbed her by the neck, and began hitting her and cussing her--a violent attack which allegedly ends in Tom raping her.
In his cross-examination of the witness, Atticus demonstrates that Mayella is motherless, uneducated, friendless, and unfairly made responsible for the well-being of the family, including her drunkard father. These circumstances have fashioned her into a very lonely woman who attempted to seduce Tom and to lie about it when this attempt was witnessed by her father.
Although Mayella's unreliable testimony and Atticus' cross-examination both suggest that Tom is innocent, the man is unfairly found guilty at the hands of the failed and prejudiced judicial system.