Interfacing MPU6050 with Arduino uno
The i2cdevlib has that declared in this file: https://github.com/jrowberg/i2cdevlib/blob/master/Arduino/MPU6050/examples/MPU6050_DMP6/MPU6050_DMP6.ino

That is the "dmp" example.
Inside the MPU-6050 sensor is a "dmp" and a FIFO buffer. The "dmp" is a processing unit that collects the sensor data, does some calculations and puts the data in the FIFO buffer. After that the Arduino is signalled with a interrupt.

In the Arduino, in the interrupt routine, the flag 'mpuInterrupt' is set. In the loop() that flag is checked and the amount of data in the FIFO buffer is read. When there is enough data, that data is read and used in the sketch.

Since the variable 'mpuInterrupt' is used in a interrupt routine and in the loop(), it is made 'volatile'. That tells the compiler that the variable can change at any moment, so the compiler can keep that in mind when creating binary code for the loop().