DOI stands for "Digital Object Identifier." You will find more and more articles and other items online using them. They provide a stable "address" that can be used to access the item. The links on this page will help you learn more about them.
The "official" source for information about DOIs, including: an Introductory Overview, a long list of Frequently Asked Questions, and the complete DOI Handbook (note: this link uses a DOI).
CrossRef is the official DOI link registration agency. The handy "DOI Resolver" will convert a DOI into a URL for you. Don't understand all this? The site will explain!