Contains utilities to interact with the MPU9255 chip which is a common 9 axis Motion Processing Unit packing gyroscope + accelerometer + compass in a single chip. Various vendors sell breakout-ready boards with the MPU9250 (or the MPU9255 revision) on them.
The actual chip is fabricated by Invensense. https://www.invensense.com/products/motion-tracking/9-axis/mpu-9250/
The MPU9250 register map is published here: http://www.invensense.com/wp-content/uploads/2017/11/RM-MPU-9250A-00-v1.6.pdf
A copy of the MPU9255 revision documentation is here: https://stanford.edu/class/ee267/misc/MPU-9255-Register-Map.pdf