Photoshop has many different ways of blending layers. Different blending modes determine how the pixels on one layer effect the pixels on other layers beneath it.

The Screen blending mode is available with all the other blending modes on the layers window. Screen blending mode brightens the underlying layers depending on how bright the screened layer's pixels are. If the screened pixel is black, it will look completely transparent. A white pixel will be white.

A screened black layer can be used to add lens flares and other bright things to a picture with the option to remove, filter or fade them.

For Example

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I've found a picture and created a new new layer above it. Then filled the layer with black and added a lens flare.

ORIGINAL Image	Lens Flare with NORMAL Blending
Lens Flare on SCREEN blending	COLOR DODGE blending

Now adding any filters to the lens flare layer will only effect the lens flare. There is now also the option later on to delete the lens flare layer or change it, offering more flexibility to your photoshop files.

Conclusion

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Different blending modes offer a different effect. Use the one which stimulates your optic nerves in the best way.