DOI stands for Digital Object Identifier. It is a unique string of numbers and letters that is used to identify content and provide a persistent link to its location on the Internet. A publisher assigns a DOI when an article is published and made available online.
APA, ASA, and Chicago styles recommend that you include the DOI when you create citations for electronic sources (and print if possible). When a DOI is available, it is generally displayed on the article record in the library's databases and/or on the article PDF.
You can also try looking up the DOIs for your references on http://www.crossref.org, which is a database of DOI information. If the DOI is not in CrossRef, you may need to go back to the original database from which you retrieved the article.