This is where you shovel dirt in. At the bottom just above the metal chute, is an opening that can be closed, adjusted, or opened completely by a metal handle (can’t be seen) at the top of the photo. The dirt falls to the riffles and while the bellows is pumped by the wheel the dirt vibrates and hops over the riffles. Can’t see the riffles because they are below the hopper. The heavy stuff stays behind.The screen can be removed if something undesirable gets in the hopper.
I have published a few articles on looking for gold. The links are here: http://john000.hubpages.com/hub/Where-to-Find-Gold-if-You-are-a-Newbie-Gold-Prospecting-the-Best-Places-Using-Gold-Panning