Just the basic information about your parent's identity, like their name, date of birth, and social security number; their living situation, marital status, state of residence, household size, and financial circumstances.
How should I respond if they are separated?
That would depend on if your parents live together or not. If your parents are married but are considered legally separated by a state, or if they legally married but have chosen to live separate lives then they are considered separate, as if they weren't married.
What if my parents don't want to provide the required information for the FAFSA form, can I submit it without that info?