The KCL standard library
Contains frequently used constants, functions for interacting with the KittyCAD servers to create sketches and geometry, and utility functions.
The standard library is organised into modules (listed below), but most things are always available in KCL programs.
You might also want the KCL language reference or the KCL guide.
Modules
appearance::appearancearraymathsketchsolidsweep::sweeptransformturns::turnstypesunits::units