DOI stands for Digital Object Identifier. It is a unique alphanumeric string assigned to an article; think of it like an article's student ID. DOI is a more reliable way to link to articles than the URL, which can change.
The information that you need to correctly generate a citation, including the name of the database, should all be available on the detailed results page. If it is missing, you may need to do some sleuthing to find what you need. The librarians are happy to help with that!
If there is no DOI, you will need to use the URL for the article. To get a reliable URL, use the permalink option in the right column. This will generate a permanent URL that will show beneath the search boxes.