is usually used when you create a macro in the code module for a worksheet, workbook, etc, when the macro is created in response to one of Excel's events (e.g. Worksheet_Change when some one updates the worksheet).
Application.EnableEvents = False
The code has the effect of telling VBA not to respond to events that occur, so if in the example the worksheet is changed again after that code has been run, the Worksheet_Change event macro will not be run.
It is usually added in code when you have responded to (say) a change in the worksheet and your program will perform some other actions on the worksheet. If you don't set EnableEvents to False, every time your macro updates the worksheet the Worksheet_Change event macro will start again. This may result in the program doing something unexpected, but even if not it gets horribly complicated trying to work out where the program will go next.
It is worth noting that when you use this command, you need to explicitly turn on events processing again with
as this is not automatically reset at the end of the program, unlike ScreenUpdating. When developing a program containing these commands, if your program errors after "= False" and you end the program, you will need to manually reset it in the Immediate Pane before the events macros will be triggered again.
Application.EnableEvents = True
Hope this helps.