Composer is Singular's overlay builder. More specifically, it is the authoring environment used to create Singular HTML5 overlays. It is web-based like all of Singular's tools and allows a user to create overlays and define how they are controlled through Singular Control Applications. You can create brand new Compositions or open Marketplace themes within Composer to make edits/adjustments.
Singular Composer UI
Visit this section of the Support Portal for tutorials on Composer.