It's a Digital Object Identifier -- a unique alphanumeric string that identifies articles, ebooks, standards, datasets, etc., and provides a persistent link to its online location.
Sanico, G., & Kakinaka, M. (2008). Terrorism and deterrence policy with transnational support. Defence & Peace Economics, 19, 153 –167. doi:10.1080/10242690701505419
It makes citing easier for you, and finding references easier for your reader.
You can put a DOI into a Web search and find the item it's linked to.
If you're referencing something you got from a subscription database, you don't need to worry about including the URL to the journal, just add the DOI.