Some authors have been quicker than others to recognize the current importance of the DOI prefix in their academic references. What is a DOI, you may ask? It stands for "digital object identifier", and is an alphanumeric string attached to an academic article, for example, https://doi.org/10.1037/arc0000014. Using the DOI system, academic articles are able to reference each other in a continuous chain of stable hypertext links, unaffected by the instability of internet addresses over time.
These days, a DOI is available for almost any peer-reviewed journal article, even ones that are decades old. Major reference formats such as that of the American Psychological Association (APA) and the Chicago Manual of Style (CMOS) now require that DOI be included wherever it is available (in the rare case of a peer-reviewed article without an assigned DOI, a stable html link should be provided instead).
The DOI is appended at the end of an academic reference; see for example this article cited in the APA style:
Morey, C. C., Cong, Y., Zheng, Y., Price, M., & Morey, R. D. (2015). The color-sharing bonus: Roles of perceptual organization and attentive processes in visual working memory. Archives of Scientific Psychology, 3, 18–29. https://doi.org/10.1037/arc0000014