Are you a student living on campus? Do you work abroad? If so, you’ve probably asked someone to collect your mail for you until you return home. The In care of field allows you to record the name of that person. Unless you move frequently or you spend a lot of time away from home for work, you likely won’t need to provide this information.
Remember, if you enter someone’s name in the In care of field of your return, the mailing address you enter should be the address of the person looking after your mail.