A "periodical" is a publication that appears at regular intervals (or periodically). This could be daily, weekly, monthly, quarterly or even annually. (Some libraries refer to them as "serials" because they're issued serially, that is, one after the other). A newspaper is an example of a periodical published daily (see "Newspapers" under the Primary Sources tab). Scholarly journals are often published monthly or quarterly (see "Scholarly Journals" under the Secondary Sources tab). A magazine is usually published weekly or monthly.
Magazines published at the time of your research interest can be invaluable primary sources. Magazine articles generally cover a wide variety of topics of interest to people of the time or that deal with current events of the day. This can provide you with insight into how people learned about events as they were happening and how these events were framed and understood by writers, journalists and others.
There are a couple of ways to track down magazine articles from the time period you're researching.
In Summon, search for your topic. When you get your search results, look at the options under "Refine your search" in the left-hand menu. Under "Content Type" select "Magazine Article".
That will remove books, newspapers, scholarly journals, etc., from your list.
Then under "Publication Date" type in the date range covered by your research.
Only magazine articles published during that time should remain for you to look at.
You may also locate magazine articles relevant to your historical research by using one of the library databases listed to the right. Each database covers a different set of publications, different time periods, and different geographical areas. Read the descriptions to see which might be of most use for your particular area of interest.