The "DOI" you might see in a reference stands for Digital Object Identifier. DOIs are assigned to articles when they are published, and they can help to track and article through its publication lifetime (i.e., wherever it may be moved over time, such is if an online journal changes its domain name, goes out of business, etc.). The DOI number is assigned by a registration agency like CrossRef, which provides a citation-linking service. Note, you will not see a "DOI" with all digital resources/articles you use. If it is not there, don't worry. If it is there, you can include it in your APA reference.
For example:
Herbst, D. M., Griffith, N. R., & Slama, K. M. (2014). Rodeo cowboys: Conforming to masculine norms and help-seeking behaviors for depression. Journal of Rural Mental Health, 38, 20–35. doi:10.1037/rmh0000008
This example is from the APA Style Blog, and you can learn more here: http://blog.apastyle.org/apastyle/digital-object-identifier-doi/