DOI stands for Digital Object Identifier. Electronic versions of articles get moved from one platform to another and URLs can change over time. DOIs offer a consistent identifier of an article so that regardless of where the electronic version "lives," you can find it with the DOI.
Most databases will include DOIs, but depending on how you found the citation, it *may* not be included. To find a DOI, input citation information into the DOI Lookup. Please note that not all articles will have a DOI.