Satin is a simple library allowing modders to add interesting graphical effects to their mods. Its main feature is to significantly simplify the use of Minecraft's data-driven shader system (that is, the one used by super secret settings, mob vision, and spectral arrows).
A full feature list with installation instructions is available on the github repository : https://github.com/Ladysnake/Satin/
You can find documentation for the shader format on the wiki : https://github.com/Ladysnake/Satin/wiki/Data-driven-shaders
Can it replace Optifine ?
No. Optifine shaders are significantly more complex than what this library allows. For a work-in-progress alternative, see the Canvas Renderer project.
Can it bring back Super Secret Settings ?
Satin is just a library, so it does not add any content by itself. You may however be interested in Secrets No More, a mod using Satin to do exactly that !
Does this library have a Forge version ?
No, and I do not want to port it myself. I will however support anyone who does so. Also note that the documentation for the shader format stays applicable regardless of the mod loader being used.