Demographics refer to the study of human population groups. Commonly collected information includes age, gender, income, and race. What's collected depends on who's collecting the data. For example, whether you rent a home or are buying one may be of interest to a realtor. Car sales staff may want to know if you buy new or used.
Health care studies look at age, gender, weight, level of education, and activity level. They may also look at marital status, living situation, social support, and income level.
Chronic back pain has been strongly linked with psychosocial factors. Demographics collected might include level of education, income, and family make-up. Some studies have shown job demands may be linked to work-related use of health care services and sick leave for back pain. These factors include level of supervisory support and control over job tasks.
With this information you should be able to look the article over and find out which demographics were reported.