Transparent Pixels Lock
This locks the transparent pixels of the selected layer. Transparent pixels means there are no any image data in those pixels. By locking the transparent pixels you can copy, move, edit pixels only inside the area of image pixels of that layer.
Image Pixels Lock
Image pixels means pixels that contain details of the colors in such layer. By locking image pixels in a layer you can work intacting those pixels.
Position Lock
You can use this lock to prevent a layer to move. Locking the position is help while selecting multiple layers using the Ctrl+Drag using the Move Tool.
You can also use the Lock all button to apply all of the above three locks at once to a layer.