The Screen blending mode is the opposite of the Multiply mode in that it multiples the inverse of the base color with the blend color. What this means is that your image will get lighter overall. In areas where the blend color is black, the base image will be unchanged, and in areas where the blend or base color is white, the result will be no change. Dark areas in the base image will become significantly lighter, and bright areas will become only slightly lighter. Adobe's user guide describes this effect as being similar to projecting multiple photographic slides on top of each other.
The Screen blending mode can be used to correct an underexposed photo, or to increase detail in the shadow areas of a photo.