A Digital Object Identifier (DOI), is an ID number for a specific item such as a journal article. It's a shortcut: instead of needing to write out a full citation for an item, you can just use the DOI to keep track of it. Some citation styles include DOIs in article references, and you can search for articles using just the DOI. You can usually find a DOI at the top of an article, with the title, author and other journal-related information.