Index issue

When I tried to create an index, I got an error like that:
Server Msg 1946,Level 16,Stage 1
Operation failed The index entry of length 1656 bytes for the index "MessageDes_Index" exceeds the max length of 900 bytes

What does that mean? How to fix it?
Thanks a lot.bump|||check this out :

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/architec/8_ar_da_0ldf.asp|||Thanks .