What are the crew holding here and its purpose?

Yup it's a big board to reflect light. It makes it nice and soft.

You don't need to stress too much about knowing all the specifics of lighting unless you want to be a cinematographer.

http://en.wikipedia.org/wiki/Reflector_(photography)

Its called a bounce card(or reflector). Its purpose is to basically "bounce" light onto the subject. Let's say you're shooting under harsh lighting conditions and the sun casts shadows on your subject, you would use the bounce card (pref. under this situation) to even out your subjects lighting. Make sense?

It looks like beadboard. There are several things we use to bounce light. Showcards are simple art cards, white on one side and black on the other. The next thickness up is foamcore - both of these are available from art supply stores in small sizes. Film supply places sell them in 4'x8' sheets that we usually cut in half to 4'x4'. Bead board is actually insulation panels made of Styrofoam with a silver paper lining on the back. It can be bought at a hardware or building supply store and cut down. once you cut it, it flakes and begins to fall apart. So the standard practice is to use black gaffers tape around the edges after you cut it to the size you want. We use bead board outside because it is stiffer than the other two and easier to hold still in a breeze. It also produces a softer reflected diffused light.

I wrote a lot about bouncing light and working outdoors with sunlight in my new book on low budget lighting "Lighting for Cinematography" - available from amazon. Check it out if that's something you're interested in. Good luck with all your shooting projects.