'Periodical' is an umbrella term for newspapers, magazines, journals - any title published regularly or irregularly. The Library subscribes to thousands of scholarly journals both in print and (mostly) online. You will come across journal articles many times in the course of your studies; they may appear on your reading list and your lecturers will encourage you to consult journal articles when you're doing research, particularly when preparing your literature review.
Journal articles are excellent sources of scholarly information because they can provide very in-depth, up-to-date coverage of a subject; they are typically very structured and written by experts in a given field. Crucially, like books, journal articles that appear in scholarly academic journals are "peer-reviewed" which means they have been critically reviewed and evaluated by a panel of experts before being okay-ed for publication. All of this means that you (as researchers) are assured of the excellent quality of information in a published journal article.