AppleScript is a language that predates the Mac as we know it today, originating in 1993. While other scripting languages have more fully featured libraries and more robust syntax, the power of AppleScript is for automating Mac applications.

Any Mac application can include a scripting dictionary, and many that ship with macOS do. A scripting dictionary lets scripts know what items the app contains, and what actions can be performed.

A script can then use that information to automate repetitive tasks, and to form a "glue" between different apps, forming fully customizable workflows.

AppleScript User Interaction is part of Standard Additions. You can find the full documentation in the dictionary StandardAdditions.sdef through Script Editor's Open Dictionary.