This button saves all the information you have added into your first plan. Once you click this button, you will be given the option to save your work to date so you may finalise it later or to save all your bills when you have completed your First Plan, in which case you will not be able to edit your First plan again.
All information in your First Plan will be saved to your account and in future when you wish to make changes you can do so in Manage My Bills or My Settings and My Alerts.