If you open a Google Doc or Spreadsheet you will find a new option in the menu bar named Add-Ons. An Add-On is a way to add functionality to Docs and Sheets through integration with 3rd party apps. It's similar to the Google Chrome Apps store.
There are many useful tools to Add-On to Google Docs to improve the efficiency of working in Google Docs. You'll find Add-Ons to help you get your own work done, and also for use in the classroom.