A C++ library that takes GLSL shaders, does some GPU-independent optimizations on them and outputs GLSL back. Optimizations are function inlining, dead code removal, copy propagation, constant folding, constant propagation, arithmetic optimizations and so on.
Quite a few mobile platforms are weak at optimizing GLSL shaders; and unfortunately they also lack offline shader compilers. So using a GLSL optimizer offline before can make the shader run much faster on a platform like that.
Almost all actual code is Mesa 3D's GLSL2 compiler; all this library does is spits out optimized GLSL back.