I hope I have edited your question correctly, and you are talking about when Atticus tells Jem and Scout that Aunt Alexandra is coming to live with them for a little while. Atticus needs Aunt Alexandra's help because he is going to be tied up with the trial of Tom Robinson. This will cause him to be away from home a lot, and he needs someone there all the time to watch and protect the children. Calpurnia will also be there, but Aunt Alexandra will be running the household while Atticus is working on the case and defending Tom Robinson.
The children aren't particularly pleased with the news. Aunt Alexandra is a little stuffy and "prissy" and not quite their style. She will expect Scout to act and dress more like a girl, attend afternoon teas, and she will definitely keep them from running and playing around the neighborhood. She will rein them in more and be a lot stricter than Atticus. She also doesn't have the accepting values that Atticus has towards people. In the end, we do see how loving and caring Aunt Alexandra is towards the children.