Voids are the spaces in between the atoms in a protein.
The "Show voids" view option represents voids as translucent red spheres.
Normally, voids occur on the inside or core of the protein, but "Show voids" can sometimes show them on the outside or surface of the protein as well, usually at concave spots. These exterior voids are just the result of how Foldit tells the inside from the outside.
In some ways, clashes are the opposite of voids. Clashes occur when atoms in the protein are too close together.