OK, without going into the 'Techy' side of things they are basically sensor bias off set values.
What you are looking for is (at rest with the aircraft on the ground):
Gyro : 0.00
Accelerometer: 0.99-1.02
Compass: 1400-1600 (can be slightly outside that range without problems but the value will fluctuate)
These value fields can be found on the sensors page in the app.