What is the Gy-85 sensor?

GY-856DOF 9DOF IMU Sensor Module, i.e Nine-axis module (three-axis gyroscope + three-axis accelerometer + three-axis magnetic field)

it is a Motion Sensor. senses acceleration and tilt.

it used with Arduino but not sure exactly what it is for 

The GY- sensor is used for measuring acceleration, orientation and Earth's magnetic field on three axis X , Y and Z.

You can use it in many orientation and motion sensitive applications, like Drones and Robotics.

Sensor Module for Arduino 

I apologize for the answer, I leave the answer to experts specialists in this field that's not my specialty field