GY-521 MPU6050 6DOF 3-Axis Gyroscope and Accelerometer Module is a carrier board based on MPU-6050 sensor which contains an accelerometer and a gyroscope. MPU-6050 sensor contains a MEMS accelerometer and a MEMS gyro in a single chip. It is very accurate, as it contains 16-bits analog to digital conversion hardware for each channel. Therefore it captures the x, y, and z channel at the same time. The sensor uses the I2C-bus to interface with microcontrollers.
The MPU-6050 always acts as a slave to the Arduino with the SDA and SCL pins connected to the I2C-bus.
But beside the normal I2C-bus, it has it's own I2C controller to be a master on a second (sub)-I2C-bus. It uses the pins AUX_DA and AUX_CL for that second (sub)-I2C-bus.
It can control, for example, a magnetometer. The values of the magnetometer can be passed on to the Arduino.