Scripting Additions are code resources (generally written in C/C++) that expand the normal abilities of AppleScript by adding commands and functions to its language…think of them in the same way that the Extensions in your Extensions folder (pre-OS X!) add functionality to your computing experience so Scripting Additions add functionality by adding helpful words, commands, and expresions to the AppleScript dictionary.
A scripting addition might be very limited in scope, or it may be a “suite” containing dozens of additional functions/commands.
Scripting Additions reside in the Scripting Additions folder (see “How do I install a scripting addition?” below). Once installed in the such a folder, they behave like an added command (or commands) in the AppleScript language.
Example:
You will have a very hard time trying to set the sound volume in your system… Unless you employ the Jon’s Commands Scripting Addition, by Jon Pugh. After installing this addition, you have access to its command set sound volume to, which allows you to moderate the sound volume (now, built-in feature in Standard Additions osax).
Scripting Additions allow you to do things which would be impossible (or at least contrived!) using normal AppleScript methods.