I485: Request for Additional Evidence

Just received this email:
Current Status: Request for Additional Evidence Sent
On January 21, 2009, we mailed a notice requesting additional evidence or information in this case I485 APPLICATION TO REGISTER PERMANENT RESIDENCE OR TO ADJUST STATUS. Please follow the instructions on the notice to submit the requested information. This case will be in suspense until we receive the evidence or the opportunity to submit it expires. Once we receive the requested evidence or information and make a decision on the case, you will be notified by mail. If you move while this case is pending, call 1-800-375-5283 to update your address.


What does it mean? What kind of evidence of information will it usually require? I haven't got the letter yet, would like to get prepared earlier.

I'd appreciate help.