What is a "Mesh Space Rotation Blend" in "Layered blend per bones" node?

In the animation blueprint, I use the “Layered blend per bones” nodes to layer two animations together. My character had a running animation and also holds an AK 47. But due to the running animation, the weapon moves jitters a lot when moving left or right. But, this “Mesh Space Rotation Blend” in “Layered blend per bones” node fixes this issue. I am a beginner in Unreal Engine 4 and I follow a series of FPS tutorials from “DevSquad”