Luminosity masks are the key feature in Photoshop that allows you to make simple controlled tone based adjustments to your images. These masks offer a quick and easy way to select only the specific tones that you want to modify, and then an easy way to modify them without effecting the rest of the image.
This can be a great asset when trying to modify images with complex tonal ranges, or lots of mixed light that can make it hard to modify without negatively effecting other parts of the image. These masks also give you the ability to change the light of an image in a way that means you can really modify an image and mold it into your vision.
Here is a great video detailing the basics of Luminosity Masks in Photoshop:
If you are looking for a more advanced tutorial, the video included below offers some great techniques and uses for a Luminosity mask. These techniques are especially useful for Landscape and Architecture photography, but also has its uses in other genres as well. Enjoy!