Parent Goals for Projects:
When you create a child goal the parent will automatically become a weighted number from the child goals. You will not be able to update the progress of the parent goal outside of updating the progress of child goals.
Parent Goals for Metrics:
When you create a child metric the parent can be chosen as a roll-up Metric from a calculation of the child goals. By default, they will be as per the parameters of the goal you have set up, you must change the option within the edit section on the child metrics to change this to a roll-up metric.