Shelf exams are a set of exams you take after certain required 3rd year rotations. You take this shelf exam after your Medicine, Surgery, OB/Gyn, and Peds(?) rotations. I get the sense from 3rd years that they are important depending on who you talk to. These exams factor in to make up part of your grade in the rotation, so if you want to gun and get honors in the rotation, you better study hard. If you simply want to pass, apparently being awake for rounds and studying here and there will do just fine.
I forgot to mention that the Shelf Exams are standardized nationally for whoever takes them at that time. Take my word with a grain of salt at this moment; everything I told you is hearsay and all my knowledge about them is based on what I have heard at this point. I'm not yet a third year, so if others chime in and agree (or not), that will give you a better idea.