What is a DOI?
DOI= Digital Object Identifier
A DOI identifies an individual article (like a serial number) such that a permanent URL can be created.
Example of DOI: 10.1000/182
The first part before the / is the publisher portion of the number. The second part points to the journal and article specifically.
When you have a DOI you can create a persistent, permanent link to an article, rather than posting full text. Why would you want to use a DOI?
Copyright restrictions might prevent you from posting the full text of an article online. Or you may only have the DOI and not know more information about the article (author, title, etc.). If you have the DOI and nothing else, then you can go here to resolve the DOI.