A dough proofer is a chamber used in baking that encourages fermentation of dough by yeast through warm temperatures and controlled humidity. The warm temperatures increase the activity of the yeast, resulting in increased carbon dioxide production and a higher, faster rise. Dough is typically allowed to rise in the proofer before baking.
If you wish to use your cabinet to proof, you will need to have the selector switch on "proof" and make sure you have distilled water in the pan.