One of the important factors for humans or computers trying to fold proteins is achieving compactness. A newly-formed string of amino acids in a cell has a preferred three-dimensional shape (its “native structure”) and the components in the interior of the folded molecule like to be snugged up together tightly, without spaces between them. In Foldit, you can get some idea of whether your protein has such spaces, called voids, by selecting “Show Voids” from the view options menu. Few players use this view to fold, but it's useful checking the quality of a fold.