Questions related to Gmail Add-ons, the modern developer platform that allows custom sidebars and buttons to be added to the Gmail user interface.
A Gmail Add-on is a standalone Google Apps Script project. Its purpose is to improve productivity and help the user perform an action directly from the Gmail page without leaving it.
The language used, Google Apps Script, is a server-side scripting language based on JavaScript 1.6. All add-ons use a "card"-style UI, to enable a common appearance on all platforms.