It is where you create the 'window' for static addresses. Note the Starting IP Address. You create a larger address reserving places for your static addresses below.
With the WRT54G I am not aware of a facility to manage the addresses so I just use a text file for that making sure I stay out of the range locked out from this screen.
So if you set the above address to 192.168.1.120 you could have 20 static ip addresses you could dispense starting at 192.168.1.100 to 192.168.1.119