Digital Object Identifiers or DOIs are codes that point to digital content permanently. They are used to point to online content in a way that lasts even when web links change. When the website hosting a digital object changes, the DOI is updated to point to the new location. If you have ever searched for articles or other content based on the web addresses (URLs) in a syllabus or bibliography, you have probably gotten a "404 not found" type of error indicating those links are no longer there. Since we want articles, datasets, and other research products to be with us long into the future, we need a pointer that is more persistent than changeable websites. That's why we use a DOI instead of a traditional web address.
One of the most important considerations for making research findings Findable, Accessible, Interoperable, and Reusable (or FAIR) is using persistent identifiers for disseminated objects. Everything in FAIR depends on Findability, and Findability depends on PIDs. You should make sure that any digital book, article, dataset, preprint, presentation slidedeck, poster, or other digital dissemination product has a DOI.
If you do not want to post the content, you can still post an abstract with a DOI and clear directions on how to arrange for access to the data or content.
There are many ways to get a DOI. Two great starting places are Scholars Compass for text and media (article preprints, conference slides and posters, and Open Access postprints), and Open Science Framework for datasets, protocols, and code. In either repository, there is an option to request a DOI.