Document Object Identifier is an alphanumeric string to identify an object and it?s persistent link over the internet. Consider it a linker shorter service but primarily used by the Academic Publishers.
The DOI for a document remains fixed over the lifetime of the document, whereas its location and other metadata may change.
Referring to an online document by its DOI provides more stable linking than simply using its URL because if its URL changes, the publisher only needs to update the metadata for the DOI to link to the new URL.
Project Page DOI Identifier Plugin