We were required to take the psychiatry shelf at the end of the Fall semester. This was the culmination of the psychiatry thread that was integrated into the CVPR, Neuro, and DEMS blocks. Shelf exams are standardized national exams in a particular medical specialty (e.g. psychiatry, OB/GYN, etc...) that are usually taken after a third-year or fourth-year clinical rotation.
It's fairly unusual for second-year students to take a shelf-exam. I haven't heard of other medical schools requiring second-year students to take a shelf-exam. The reasoning of our course directors, I think, was to provide us with a sneak peek into what kinds of tests we'll be taking once we start clinical rotations. Maybe they also wanted to give us a reason to start studying psychiatry for Step 1.
"Shelf exam" sounds kind of scary, but it was actually very low-pressure because the test doesn't contribute at all to our grade or academic record. There are no negative consequences from bombing this test, but someone who does well could be eligible for some sort of scholarship.
To prepare for the psychiatry shelf exam, I just read through Psychiatry Blueprints and did practice questions, both at the back of this book and through USMLE World Qbank. I found this to be sufficient preparation.