Screen is the opposite of Multiply. Rather than creating a darker image, you create a lighter image. Photoshop and BodyPaint look at each channel's color information and multiplies the inverse of the blend and base colors. The result color is always a lighter color. Screening with black leaves the color unchanged. Screening with white produces white.
Screen is useful for creating glows, retaining just the light colors in a gradient, and creating light noise effects such as snow and stars.