MPU6050_6Axis_MotionApps20.h

I want explanation function in MPU6050_6Axis_MotionApps20.h or DMP class reference such as mpu.dmpGetQuaternion(&q, fifoBuffer);
mpu.dmpGetGravity(&gravity, &q);
mpu.dmpGetYawPitchRoll(ypr, &q, &gravity);

Explanation of the use of the address-of operator, or the word "quaternion", or what is yaw/pitch/roll or gravity?

I want to know how does it work of each command in this library similarly to https://www.i2cdevlib.com/docs/html/class_m_p_u6050.htm.Sorry I'm not good in English.