Gravity is a type of acceleration, this is what the sensor gives you.
Acceleration is a vector value, because it has a direction. The length of the gravity vector is always around 9.81 becahue that's the average gravity (of course it can be different, for example on the Mount Everest you have a smaller gravitational force than on the bottom of the see)
If you want the ball, to always fall to the ground with an acceleration matching the gravity, then using the acceleration sensor is not enough. If you start waving the device around, then the acceleration will change. The accelerometer also doesn't register anything when the phone is freefalling. If you don't care about this, then you can skip the next paragraph.
You should instead use the orientation of the device, create a unit vector out of that and multiply it with 9.81.
Then, to apply this to an object, you need to increase it's velocity by this vector multiplied by the amount of time that's passed between calls (time should be in seconds, with 60 fps it should be around 0.016666 seconds)