"Shell" is a Microsoft Access Function. The following is quoted exactly from the Help facility from within Access. You can refer to it for any further usage instructions or clarification.
From Microsoft Access Help utility, and I quote:
Runs an executable program and returns a Variant (Double) representing the program's task ID if successful, otherwise it returns zero.
Syntax
Shell(pathname[,windowstyle])
The Shell function syntax has these named arguments:
pathname
Required; Variant (String). Name of the program to execute and any required arguments or command-line switches; may include directory or folder and drive. On the Macintosh, you can use the MacID function to specify an application's signature instead of its name. The following example uses the signature for Microsoft Word: Shell MacID("MSWD"
windowstyle
Optional. Variant (Integer) corresponding to the style of the window in which the program is to be run. If windowstyle is omitted, the program is started minimized with focus. On the Macintosh (System 7.0 or later), windowstyle only determines whether or not the application gets the focus when it is run.