DOI stands for Digital Object Identifier. A DOI is meant to be a permanent "address" for an online resource, such as an article or ebook. It can be written as DOI: [number] or https://doi.org/[number].
The DOI is usually easy to find in database search results:
Note: Google Scholar does not show DOIs in search results.
The DOI is also typically shown on the first page of a journal article:
You can also go to http://crossref.org/, click on "Search Metadata," and search for an article there.
If you are not able to find the DOI using any of these methods, the online resource probably does not have a DOI.
DOIs were invented in 2000, so anything published pre-2000 will not have a DOI. Some articles published after 2000 will also not have a DOI. DOIs are becoming more common in recent years.