DOI is digital object identifier. Most articles have one, similar to your social security number or student ID number. It is unique and persistent and thus helps locate the exact article in a collection of similar names or titles.
It is often part of the URL (link), such as https://doi.org/10.1109/5.771073 where you can see an article describing DOIs and listing its own DOI in the metadata (description).
Use the Google Scholar box to search for full text when you know the DOI. It might look like: 10.1109/5.771073 (from the example above).