Before attempting to fly your multirotor copter for the first time you should ensure you have a calibrated ACC sensor. We calibrate the ACC sensor during testing using MultiWii Config but calibration may have changed if you've done a firmware update. In any case, it's just good practise to verify your Quadrino's accellerometer is properly calibrated.
Place the board or quad flat on a table, plug the Quadrino into the USB connection of your computer. Now startup the MultiWii Config application, select the COM port from the left hand side, and click the START button. Now look to the left of the graph output, the ACC sensor readings should read near zero for PITCH and ROLL, and close to either 256 or 512 in the Z axis. Close would be anything within about 10 points. If you are not withing this range then recalibrating the sensor is necessary.
It is used to detect the attitude (orientation) of the copter with reference to the ground by sensing the direction of gravity. It is the same process we use to stay upright when we close our eyes. In our case there are organics in our inner ear that act as accellerometers. On the Quadrino, there is an electronic chip that contains a micro-mechanical (super super small) machine that can measure the direction of gravity in all axii.
Detecting the orientation of the copter is very important for keeping the copter steady. Although it is specifically required for the auto-level flight feature, it is also used in normal gyro (acro) flight mode as well, though it is not required. A multirotor copter can function using only a gyro sensor, but with both sensors a fusion of the two sensors occurs that results in a more accurate and precise measurement of the copter's attitude (orientation).
The ACC sensor is also required in the altitude hold feature. In this case, it is used to estimate the velocity of the copter in the vertical direction. It is used to estimate the velocity by performing integration (calculus) on the ACC sensor readings in the Z axis. This process is prone to noise and numerical error and this makes altitude hold the toughest feature to implement on multi-rotor aircraft!
Calibration of the ACC sensor is a very simple process. To calibrate keep the board very still and level on a flat surface such as a table. Click the CALIB_ACC button above the graph section and wait a few seconds. You will notice the blue line in the graph section dip for a moment, then return to normal. The ACC sensor readings should now be close to the values in the above paragraph, so the ACC PITCH and ROLL near zero, and Z near 256 or 512. The Z value of 256 or 512 will depend on what sensor and firmware version you are using, both values are valid.