Add-ons enhance your Google Apps (ie. Docs, Sheets, Slides, etc). They vary between your Google Apps. So, the add-ons that you have available in Docs is different from what you will see in Forms and Sheets, for example.
To access add-ons from Google Docs, Sheets or Slides, click on the "Add-ons" tab in your top menu bar. Then click "Get Add-ons" and search for the different add-ons that you want to add to your apps.
To access add-ons from Google Forms, click on the snowman (3 dots) next to your profile picture, and then click on Add-ons. Then search for the Add-ons that you are interested in.
Once you have found an Add-on that you want to add to your Google Apps, click on the "Free" button to add it to your App.
Here is some information separated by Google App about different Add-ons! Click on the images below to explore the information.
Docs Add-ons
Podcast about Google Doc Add-ons from The Google Teacher Tribe
The 32 Best Google Doc Add-ons in 2017 by Matthew Guay
10 Popular Google Docs Add-ons for Teachers by Diana Clark
Slides Add-ons
9 Awesome Add-ons to Supercharge Google Slides by Control Alt Achieve
6 Great Google Slides Add-ons for Teachers by Educational Technology and Mobile Learning
4 Good Google Slides Add-ons to Visually Enrich Your Presentations
Forms Add-ons
YouTube Tutorial: 5 Great Add-ons for Google Forms from Using Technology Better
5 Awesome Add-ons for Google Forms from Technology, Education and Everything in Between
9 Awesome Add-ons for Google Forms by Control Alt Achieve
Sheets Add-ons
12 Google Sheets Add-ons to Streamline Reporting from Caroline Forsey
50 Google Sheets Add-ons to Supercharge Your Spreadsheets from Matthew Guay
Vital Google Sheets Add-ons for Better Spreadsheets by Kayla Matthews