One of the best creative tools Elements has to offer is layer masks. Masking is essentially just another way of making a selection. Instead of making a selection with a single selection outline — either it is selected or it isn’t — masks enable you to define your selection with up to 256 levels of gray (from white to black). You can, therefore, have varying levels of a selection.
Here’s how it works. First, think of a layer mask as a sheet of acetate that hovers over your layer. With any of the painting tools (Brush tool, Gradient tool, and others), you apply black, white, or any shade of gray onto the layer mask.
Where the mask is white, the image on the layer is selected and shows. Where the mask is black, the image is unselected and is hidden. And where the mask is gray, the image is partially selected and therefore, partially shows. The lighter the gray, the more the image shows. By default, the mask starts out completely white so that everything is selected and shows.