I did read the data sheet of MPU 6050 and it doesn't explain the data outputs exactly and I couldn't understand what it explains.
a/g: 216 -288 14724 -440 -16 256
These six numbers represent one reading from the device.
The first three numbers are from the accelerometer, and the last three numbers are from the gyro.
The first three numbers represent the apparent acceleration of the device in the x,y and z directions. When the device is not actually accelerating, you will see an apparent acceleration in the upwards direction. That is because the effect of the gravitational force on the sensor causes a deflection of the sensor equivalent to the deflection that occurs when the device is subjected to actual acceleration. These three numbers can be considered as a vector which shows the direction of "up". The vector is with respect to an apparent coordinate frame attached to the device ( and usually indicated with a marking on the pcb surface, although many of them are wrong ).
In the case of the reading which you have, the vector is ( 216, -288, 14724 ), which is predominantly in the Z direction. So you probably had your device almost flat on the table.