The 'P' of MBP. Stores don't always need a full casepack of something, for instance, nail polish comes in a case pack of 72, but how often is a store gonna need that much? So what is done is those bigger casepacks are opened and smaller quantities are put into a repack box with other items to be shipped to the store. My understanding of packing is that you scan a casepack, and a screen tells you how many quantities need to be packed to which stores. When the repack is full, you tape it up, label it and send it up to the mezz. Note: This only what I've heard of, as I'm in OB, so there's probably stuff I'm missing out on that your trainer/temp lead can fill you in on.
Now, things to make yourself better in packing, from my OB and a store-side perspective:
Make sure your empty boxes make it to the baler line and NOT the line going to the mezz
When labeling, make sure it's on the most conveyable side, and for repacks, that any old labels are either removed or completely covered up
Tape up your boxes good and throw away any bad repacks. OB and store-side hate nothing more than a repack being held together by a lick and a prayer that falls apart when you lift it, and don't even get me started if it breaks on the mezz...
When labeling, circle the correct zone. A zone is sorta like a dept (16 is softlines, 9 is security, 6 is office/stat, etc), and having an incorrect zone circled (or not at all) inspires a lot of hate from the guys store-side, so don't do that.