Autoexecution is used to set the operating environment or to define variables and procedures for use in the worksheet. The confirmation dialog displaying this message is launched when a worksheet containing items set to auto execute is opened.
If you click Yes, the preset items are executed. If you click No, the items are not executed.
However, if you click No but want autoexecution of these items, select Edit>Execute>Repeat Autoexecute.
The autoexecute feature is set in the Security tab located in the Options dialog.