What is SFM?

Source Filmmaker (often abbreviated as SFM) is a 3D computer graphics software toolset used for creating animated films, utilizing the Source game engine. The tool, created by Valve, was used to create over 50 animated shorts for its Source games, including Team Fortress 2, the Left 4 Dead series, and Half-Life 2. On June 27, 2012, Valve released a free open beta version of the SFM to the gaming community via its Steam service.

~ SMG4 Wiki