Ratiometricity means that zero point and sensitivity are proportional to the supply voltage. If the supply voltage is fluctuating the output will also vary. The idea is that when using the same reference voltage for both the sensor and the measuring part (A/D-converter) , an error in the reference voltage is automatically compensated for.
Ratiometric error is the relative deviation (%) from the proportional behaviour described above, i.e. if the output voltage changes 3% at a supply voltage change of 5% we have a 2% ratiometric error.