This simple question calls attention to a very complicated situation happening in the courtroom in Chapter 18 of To Kill a Mockingbird. Mayella claims to be scared of Atticus, and she is right to be scared of him, as Atticus has the power to expose her as a poor white woman who has attempted to seduce a black man.
If Atticus is successful, the truth will set off a chain reaction that can only create more problems for Mayella. It is this possiblity that scares Mayella the most: she will be cruelly punished by her father and she will humiliated when the public realizes that her need for kindness and touch drove her to try to seduce Tom.
If the truth about what happened comes to light, Mayella will likely have to endure the wrath of her father, Bob Ewell, whose reputation for viciousness is widely known. Bob will be angry with her for bringing humiliation on his name, for breaking unspoken rules around the interactions between blacks and whites in Maycomb, and for making the truth about his abusive behavior obvious.
As well, Mayella will be seen by the community as a sad victim of her circumstances, a pathetic and needy woman who wants physical affection so badly that she makes a move on a married black man. She has misjudged her power as a white woman and she has misjudged Tom; both of these mistakes have led her to where she stands now, and she is fearful of the consequences.