You will be unable to make changes to transparent, or see-through, parts of a layer.
An Alpha Channel describes and controls the opacity or transparency of all color channels on a pixel. When the Lock alpha button is turned on, it prevents the user from making changes to those transparent portions of an image. When it is unlocked, the user can manipulate the image in any way he or she likes. Transparent portions of an image can be painted upon or can be filled upon with color if this feature is turned off.
