Demographic information can include things like your age, gender, race, income, profession, number of kids, etc. You’ll also be asked lots of questions about how you use products and services, like how often you watch TV, where you buy your groceries from, etc.
If a survey asks for your Social Security number, bank account number, or driver’s license number, leave the survey immediately. This isn’t relevant information, and it could mean you’re on a spammy site.