What does Beth bring over to Laurie when he is not feeling well?

I believe you're referring to the incident in chapter 5. One day, when Jo's outside playing in the snow, she sees Laurie at the window looking kind of sad. She throws a snowball at the window to get his attention. Laurie leans out of the window and tells Jo that he hasn't been feeling well lately. Jo may be a headstrong, rebellious tomboy, but she also has a streak of kindness and feels sorry for the young man. She asks permission from her mother to go round to Laurie's house to try and cheer him up a bit. Permission duly granted, Jo turns up on Laurie's doorstep with a dish of her mother's blancmange in one hand and three of Beth's kittens in the other.