Rotation node handles instances rotation/orientation.
Static rotation rotates all instances in the same direction.
Rotation space sets what space (world, surface or object) to use for rotation.
Random rotation rotates all instances in random directions. Random rotation makes an illusion that all instances are different due to different orientation to observing camera.
Texture rotation controls instances rotation by texture around every axis.
Bump rotation rotates instances to follow bump mapping normals.
Gravity orientation rotates instances around local Z axis to keep certain instance orientation toward Z axis tilt.