Two atoms can't occupy the same space at the same time.
In Foldit, when two atoms are too close, they clash. If the "Show clashes" view option is set, clashes are represented by a spiky red ball.
Clashes push the protein apart when you wiggle it. (Specifically, the "wiggle backone" version of the tool tends to push the protein apart.) The clashing importance slider can be used to temporarily lower the amount of push.
Clashes are often caused when the protein's sidechains get too close to each other. The shake tool can resolve many clashes, and doesn't cause the protein to move apart. The "wiggle sidechains" version of wiggle can further reduce clashing without changing the overall shape of the protein too much.
In some ways, voids are the opposite of clashes. A void occurs when there's too much space between atoms on the inside of the protein.