A DOI, or Digital Object Identifier, is a string of numbers, letters and symbols used to permanently identify an article or document and link to it on the web. A DOI will help the reader of an article easily locate a document from the citation.
In most recently published articles, the DOI will be printed with the article itself, usually on the first page somewhere, or in the header or footer. Generally, DOIs will be available for scholarly articles published after 2011. Some articles before 2011 have them but many don't. You can see how to grab the DOI from EBSCO, Gale or ProQuest.
In EBSCO databases you can grab the the DOI at the bottom of each entry on your result list if it is available. Generally, DOIs will be available for scholarly articles published after 2011.
Alternatively you can grab it from the article page under DOI or from the citation option under most citation styles.