DOI stands for "Digital Object Identifier." Basically, it's a unique combination of letters and numbers that is assigned to a journal article to make it easier to find online. More and more journal publishers are assigning DOIs to each article they publish; they can usually be found in the database article record or on the top/bottom of the article itself.
To watch a short video explaining what a DOI is, click here.
APA and MLA Styles include the DOI as part of their citation formats. For an example of what a DOI looks like, take a look at the Journal Article on a Website or Database example below.