Its very simple...this may help you as you can see I have made some wings and, I am using the layer mask to tidy them up.

Basically, as previously mentioned the layer mask helps you to alter the appearance of an image without destructing it.
The layer mask is useful for removing any unwanted parts of an image. You will need to be continually altering the size and softness/hardness of your brush to access various areas of your image. By selecting WHITE from your foreground/background palette you will be able to restore parts of your image which have been accidently covered by your BLACK brush.

Lunarcore do a nice little explantory tutorial which will help you.