Voids are the spaces in between atoms in a protein fold.
To see the voids in a current solution, go to the view menu and choose "show voids." This slows down FoldIt considerably, but it can be used to see where you need to improve Compactness.
According to the developers, "a well-folded protein will have no or almost no voids."
Voids are always shown as red spheres of various sizes.