Ive not seen this on my X5, but it could be one of two things:
1. Radio anti-theft code, found in the radio/nav book/manual that came with the X5
2. User entered security code that must be re-entered to start the car (I have this feature on my 1998 E36)

Given that you aren't saying you can't start your X, nor have mentioned entering a code you can't remember, I am going to go with door number 1. Hope you have the book.

__________________
Daily Drivers:
- 2008 535i, 320,000km
- 2004 X5 4.4, 01/2004 production, 420,000km
- 1997 328i, 350,000km

Track:
- 1996 328i, track/race car, ~300,000km

Winter:
- 2013 Ski-Doo MXZ X 800 E-TEC, trail can
- 2007 Ski-Doo MXZ Blizzard 800 HO
- 2001 Ski-Doo MXZ 600 w/800 engine, exhaust
- 1978 Ski-Doo Olympique 340 (vintage race sled)
- 1977 Ski-Doo Olympique 340E