Good question.

A DOI is a Digital Object Identifier. All peer-reviewed papers published in scientific journals usually have one of these. It looks a bit like a URL.

Example, here is a paper titled "What Makes for a Merry Christmas?" published in "Journal of Happiness Studies"

https://link.springer.com/article/10.1023/A:1021516410457

There we can see it has a DOI like this ...

doi: 10.1023/A:1021516410457

It is towards the bottom of the page.

No access to article itself, paywall.

But if that DOI is entered into sci-hub ... https://sci-hubtw.hkvisa.net

There is the full paper.