Extracurricular activities are required to be a competitive applicant to medical school. The kind of activity you do is completely up to you and should be something you are passionate about so that it shows during your interview.
When they say "volunteer work," they mean the kind of work that helps an individual or a group of people, that you do without pay. For example, tutoring at an after school program, coaching volleyball to at risk youth, doing something in your community that benefits the community, etc. Volunteering is highly recommended if you want to apply to medical school. You can volunteer in an area you are passionate about, to help the group of people that you most care about.
Your clinical experience is gained by working with patients (i.e. people who are sick or in need of an evaluation). People gain clinical experience by shadowing doctors, by working in homeless shelters, women's shelters, free clinics, working as volunteers in hospitals, working at hospices, etc. etc.. The opportunities are endless, although that doesn't make acquiring these experiences easy.
Shadowing is following a medical personnel (often, a doctor) at their job. Students often shadow emergency medicine docs, family medicine docs, surgeons etc. Although the thought of approaching a busy doctor can be frightening, most of them would love to have you shadow them for a few hours. There are also formal shadowing programs that many undergraduate schools offer their premedical students. Medical schools also have programs run by the school or by the medical students themselves, that offer shadowing experiences to premed.
So, if you shadowed a doctor at a youth clinic where you watched him or her take care of the patients, and then go on to volunteer to teach the kids about teen health, you would be fulfilling all three requirements.
Hope this helps! Good luck!
