Add-ons are specifically designed for Google Docs, Google Sheets, Google Slides, and GMail. Anyone can write Add-ons that extend the functionality of the software. Sometimes these Add-ons are written to add features that GSuite doesn't have, and other times they are written to help integrate other apps with Add-ons.

From Docs, Sheets, and Slides, go to the "Add-ons" tab and start browsing!