When Tom testifies in his trial, he realizes the fine line he must walk. He must tell the truth, but because of the strong and entrenched racism in which he lives, he must be very careful to stay "in his place" and not seem "uppity." His life is at stake, and he cannot offend the white jury members. Tom is understandably scared to death as he testifies, and he chooses his words very carefully.
According to Tom, Mayella had saved money for a year to have enough so that she could send all the Ewell children to town to buy ice cream, implying that she had done so in order to be alone with Tom. He testifies that he told her she had been "smart" to "treat 'em." Tom then quickly explains his choice of words:
I don't think she understood what I was thinkin'--I meant it was smart of her to save like that, an' nice of her to treat 'em.
Tom wants to make it clear that he did not want to be alone with Mayella, which would have been a fatal admission.
Later, when being questioned by Mr. Gilmer, Tom's testimony contradicts Mayella's. Mr. Gilmer jumps on this, asking Tom if he is saying that Mayella lied. Tom responds:
I don't say she's lyin', Mr. Gilmer, I say she's mistaken in her mind.
Tom knows that calling Mayella--or any white person--a liar would surely destroy any slim chance he might have with his white jury.