Some common rejects labels are as follows:
Barcode Required Reject: A barcode that must be read on each piece was not found on this piece.
Database Reject:  A piece could not be inserted into the database.  Sometimes, the error will fix itself if the piece is rerun, generating a label.  It can often mean the account MID is incorrect or not in place and checking the account profile for that particular account and class of mail can resolve the issue.  If not, rerunning the patch and/or label list will often correct the error.
DPV Reject: The last 2 digits of the 11 digit zip code could not be determined. This typically means that the street number is wrong.
DPV FP Reject: This is a fake seeded address to catch people who are trying to figure out every address in the US.
LL Reject: The LACSLink reject is a reject that is mandated by the USPS. It deals with rural routes.
LL FP Reject: This is a fake seeded address to prevent a person from figuring out every address in the US.
MC Reject: Move Candidate Reject. The software was unable to read the recipient name on the mail piece, so the mail piece “might” be a move.
MU Reject: Move Update Reject. The mail recipient has moved to a new address.
OCR Reject: The OCR could not find a valid address after reading the mail piece.
Over Weight Reject: The piece weighs too much for the selected mail class.
SL Reject: SuiteLink Reject. This began in Cycle N. The secondary information for the street address was incorrect and must be corrected. 
Under Weight Reject: This piece weighs too little for the selected mail class. This label is very rarely encountered.

In addition to these rejects, there are additional rejects that will appear in various places throughout the software, but will not appear on any label: 

Camera Reject: The camera could not take a picture of the mail piece.
Out of Profile Reject: Any mail pieces that are not specified in the sort profile will sort to this rule.
Routing Reject: The ADC, 3DS, 5DS, etc. could not be determined for the 11-digit zip code on the mail piece.
Additional rejects, their codes, and their descriptions are listed in the files located in folder \\fs1\Customers\Customer Support Documents\Knowledgebase Articles\List of Rejects