OnCommandStateChange: Write an OnCommandStateChange event handler to manage your application controls. This event fire as the result of certain methods changes, or when the user clicking on a link.
In simple words, here is the place to Enable your back and forward navigation buttons.
For example, if you load your browser, the back button should be disable since there is no URL address to navigate back to.  After the first navigation, this button should be enable for one click only, and so on.
:-

The Command arguments is one of:
type TCommandStateChangeConstants =(
    CSC_UPDATECOMMANDS = $FFFFFFFF,
    CSC_NAVIGATEFORWARD = $000000001,
    CSC_NAVIGATEBACK = $00000002);

Details:
CSC_UPDATECOMMANDS

The state of one of the toolbar buttons may have changed. Check status of each toolbar button relevant to your application and whether the toolbar is visible.

CSC_NAVIGATEFORWARD

Indicates whether there is "forward" history.

CSC_NAVIGATEBACK

Indicates whether there is "backwards" history.

 procedure TbrowserFrm.WB1CommandStateChange(Sender: TObject; Command: Integer;
  Enable: WordBool);
begin
   case Command of
        CSC_NAVIGATEBACK : btnBack.Enabled  := Enable;
        CSC_NAVIGATEFORWARD : btnForward.Enabled := Enable;
     end;
end;