Name and description of this mod are very technical. So here's the tl;dr for whoever might read this.
What is player_monoids?
player_monoids offers a compatibility layer so that multiple mods can change physics related aspects like your character's walking speed or jump height.
What does this mod do?
Usually, effects of different mods get multiplied. So if you have a sprint mod that doubles your walking speed and also have boots equipped that make you walk twice as fast then now your speed is 4x the original speed. This mod changes the calculations so that the effect gets incrementally smaller the more you add to it.
Should I install this?
Yes. If you have at least one mod that uses player_monoids as an (optional) dependency then I recommend that you install this as well.
Humble advertisement: If you have any mods installed that don't use player_monoids but instead use playerphysics then get the "Playerphysics API with Monoid Support" mod as well. The effects of this mod will then also apply to those mods.
A very good tweak to the physics calculation, especially when using many mods.