Digital Object Identifiers are a system of recognition and location of digital manuscripts and journal articles.  They provide a consistent location for a given reference.  I (AGM) don't know why this is important, since as far as I know, the journal name, volume and page number is sufficient to locate any journal article (despite many libraries asking for more information - that extra information is merely confirmatory).

DOI INSTRUCTIONS

A digital object identifier (DOI) can be used to cite and link to
electronic documents. A DOI is guaranteed never to change, so you can
use it to link permanently to electronic documents.

To find a document using a DOI:

1. Copy the DOI of the document you want to open.  The correct format for citing a DOI is as follows:   doi:10.1016/j.physletb.2003.10.071

2. Open the following DOI site in your browser::           
http://dx.doi.org

3. Enter the entire DOI citation in the text box provided, and then click Go.
The document that matches the DOI citation will display in your browser window.

Whatever.