I can't find what you guys are talking about when you say "masking".
"In art,
masking is using materials to protect areas from change, or to focus change on other areas."
With photoshop, you use layer masking. In this case, you have 2 layers; one is the selected area (the background already removed). The 2nd would be a copy of the 1st layer. While on the 2nd layer, look at the bottom right section where the layer panel is. Hover over each until you find "add layer mask" towards the bottom. This converts the layer into a mask that you can delete/show in some parts, but not in others. Grab the brush tool and press D on your keyboard to have it reset the brush colors to black (foreground) and white (background). Now, if you paint over that layer mask with black, it will delete that portion of the layer. If you paint over it again with white, it will bring it back.
This is used for when you want to change a picture but keep some parts of it unchanged. Ex: If you took a crappy shot with not depth of field, you can copy the picture as a new layer, blur that layer, add a layer mask, and delete the blur from the subject. Don't go using this to cheat, though.
If you're still confused or want more tips IM me on AIM (Felicis90) or send me a PM.