For most people it will be things like tax documents, medical receipts (assuming you are in a country where that’s important), photos of kids’ life milestones, photos of family members who have passed away, copies of leases, receipts for large purchases for insurance purposes if your house burns down. Things like that. Also, if you do freelance work like web design, photography, video editing, writing, music production, game design, research, etc, you want to make sure that stuff is backed up.