Tagged PDF is a form of embedded metadata for PDFs that allows support for bookmarks and copy-and-paste on text, and gives a namespace for Adobe's DOM that is leveraged by such technologies as layers, interactive PDFs, and RichMedia Annotations.

FAQ

1. What is Tagged PDF?, on Stack Exchange
2. Are Bookmarks and Hyperlinks equivalent?, on Stack Exchange

Generating Tagged PDFs with a TeX engine

1. How can tagged PDFs be created that support Universal Accessibility and reflowing?
2. Can MikTeX create Tagged PDFs, on Stack Exchange

Understanding how TeX engines tag PDFs

1. How to be 100% sure what you see is what everyone else sees?
2. Revisiting producing structured PDFs from LaTeX
3. Latex tag for making a comment appear or disappear in pdf? - Dynamic PDFs depend on tagging to identify parts of the document
4. How can I get pdflatex to make tooltips attached to text, like HTML's abbr tag? - Tooltips depend on tagging to identify parts of the document

Other technologies for generating and manipulating Tagged PDFs are documented on Stack Exchange (for instance, the iText Java library) and on Super User (for instance in Microsoft Office, Libre Office, and using pdftk)