The org-link tag should be used for questions about links in org-mode files. This is a feature of org-mode that allows users to include hotlinks to org headings within the same file, to other files on the same computer (e.g. text, emails, pdfs), to urls for internet resources. Additional link types are provided by external packages.
Support for links is built-in to org-mode, and is described in the Org mode manual. Extensions are available to support additional link types, such as org-ref for citations, and org-pdftools for links to pdf documents.