Composer is the official scene (screen) creation and management library in Corona SDK (which is free to make 2D games for Windows, Mac, mobile devices, and Apple TV). The Composer library provides an easy way to create and transition between individual scenes, such as going from a main menu scene to one where the player chooses a level, etc.
This tutorial is not for complete newbies to Corona SDK -- it assumes you've played around with it at least a bit.Probably late-beginner to early-intermediate level.