Due to the significant uncertainty of the future for the family, Tom tells Ma that his attitude about going to California is going to be the same as his attitude in prison - take things one day at a time.
Unable to control or predict what is to come, the Joads will have to find a way to cope with this lack of control without becoming overly stressed. There will be enough stress in dealing with hardships when they arrive. This is Tom's philsophy and Ma agrees with Tom. She adopts the same philsophy, but, as the person the family counts on for its stability, this point of view becomes integral to her role.
Much of the stress of the family's situation falls to Ma and she struggles to maintain an equanimity and poise as the family slowly disintegrates.