Access Code is a number with a format like #1234* or #2013* or #5555*
It starts with * and ends with *
Another name: secret number
You can log in to the user control panel, go to menu Device Setting -> Sync Setting and scroll down to see the text-box like the picture below.
You also can change the access code. The purpose of the access code is: you make a call with this number (make a call and wait 1 or 2 seconds and then click stop/cancel the call) to show or to hide the icon of the application.