steering angle sensor: there is a sensor on the steering column that measures the angle of the steering wheel.
The ESP needs this to work, there are also G sensors in the car, ESP measures what the car is doing via the G sensors, ABS, throttle, and steering angle.
If any of these sensors are not working then you can get an ESP fault.
There is either a mechanical fault with the sensor, (im not sure what the second fault is) or its also possible the angle is out of calibration. (ie its not reading zero when straight) have you have tracking or anthing done before this issue came to light?
To replace the sensor its a steering wheel off job. bear in mind its not just a case of replacing the sensor, the new one will need to be set up on the car via vagcom or the like. they are also not cheap at all.