The raise and lower commands control the window stacking order. The stacking order controls the display of windows. Windows higher in the stacking order obscure windows lower in the stacking order. By default, new windows are created at the top of the stacking order so they obscure older windows. Consider this sequence of commands:
button .one frame .two pack .one -in .two
If you do this, you do not see the button. The problem is that the frame is higher in the stacking order so it obscures the button. You can change the stacking order with the raise command:
raise .one .two
This puts .one just above .two in the stacking order. If .two was not specified, then .one would be put at the top of the stacking order.