Periodicals are any publications that are published more than once: therefore they are periodical. Journals, magazines, newspapers are the most common periodicals.
If you ever find a citation or reference that you like, and you want to see if we have it, go to the periodical list and type in the name of the journal (not the article title).
You will be referred to a list of the databases that carry that journal. Note the dates available online. If your journal article is older you may have to come into the library for a physical copy.
If the library does not have access to the journal at all, fill out an interlibrary loan request.
A less commonly used function of the periodical list is searching by subject and then viewing all the journal titles associated with the subject. In real life, you will not conduct research this way. Instead use databases (JSTOR, CINAHL, etc...) and do keyword searches.