Mac Script Editor Overview (Yosemite+)
Provides nice details of the Script Editor user interface (UI), and how to get started using the editor. Helps the user get familiar with the tool. Does not go into any detail about programming.
To learn more about AppleScript and JXA, do a Google search on "AppleScript getting started" and "JXA getting started".
The Script Editor was released as part of the Mac OS X Yosemite release. It replaces the "AppleScript Editor" in previous OS. It supports programming in both AppleScript and JavaScript for Automation (JXA).