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 hurt the score, even it you've set a low value for clashing importance. (Specifically, there's a clashing component of the score.)