The term DOI stands for "Digital Object Identifier."
The DOI, when available, serves as a permanent link to the article. It is usually found on the landing page of the article in a database, or on the first page of a print article.
MLA, APA, and other citation formats require the use of a DOI when it is available.
Most, but not all, scholarly/peer reviewed articles have a DOI. Here is an example of where to find the DOI listed on the landing page of an article from a database.