at user command

i am displaying a report through a simple ALV

i called only the FM fieldcatalogs and reuse alv

i need to five the user the option to if he want to download the report in an excel he can do so.

i know that the coding is as follow.

AT USER-COMMAND.

CASE sy-ucomm.

WHEN 'EXCEL'.

/**writing a subroutine to download in excell

ENDCASE.

what i don't get is the meaning of code below what does it do. will the user click on a button if yes where will the button appear. i would like to write the program myself but i'm unable to do so could anyone give me the picture view plzzz

CASE sy-ucomm.

WHEN 'EXCEL'.