What’s the difference?

When your skin is damp, it will allow more water into your skin, and it will make your skincare products more potent which can either be helpful, or harmful

When should the skin be damp, and when should it be dry?

Your skin should be damp before using hydrating ingredients, such as hyaluronic acid, glycerin, and overall hydrating products. And it should be dry before using retinoids or exfoliating acids, as it will make it more potent and more irritating.