Academic journals are published one or multiple times per year. Issues are grouped into volumes. (Volume 3 of a journal might contain 1-4 issues.) Articles in academic journals are peer-reviewed, meaning that they have been read by experts in the field who advised the author on how to make the article stronger, more accurate, or more helpful to readers.
Journal issues might have special themes, meaning all the articles in one issue deal with a central topic.
(All journal descriptions are from Scimago Journal & Country Rank. )