Cubemaps are textures game developers use all the time to simulate reflective surfaces without the heavy rendering cost of accurate reflections. They've been here for a while - even The Sims 2 had cubemaps!
Cubemap implementation on The Sims 4, however, is far from perfect. They are fixed textures with no association to environment or light level, and surfaces appear to reflect more light in darker rooms.
This mod replaces the cubemaps with blank textures, disabling the effect.
It's not a perfect solution as some objects look plasticky without the fake reflections, but overall there's an improvement to the game's visuals, especially in dark indoor areas.