Add-ons are scripts that expand the functionality of Google Docs and Forms / Spreadsheets. While there are a number that can be used with forms, there are two key ones:
Multiple choice or exact text answer, check for understanding tools.
Provides the ability to collect data in a form and then create nicely formatted Google Documents with the data. The Docs can be shared back with the contributors or not.
Autocrat Website - Includes some a couple videos that go into more depth.