What exactly is medical interning?

Is medical interning the same as volunteering at a hospital. Could you call that medical interning? What is the youngest age to start, and what exactly do you do? I'm only 15, about to be a high school sophomore. I'm hoping I could do something health-wise during this summer to put on my college resume. I doubt I can do a internship though since it's about to be July, so what are some other options? Just volunteering? Or maybe something else productive? Doesn't have to be health-related. Just impressive :) Thanks