Line regulation is the ratio between output voltage changes to the input voltage changes, as an percentage, i.e.
Line regulation = (Change in Vout) / (Change in Vin) X 100%
It's an essential specification on power supply and voltage regulation circuits. For example, if the input voltage changes by 10 V and the output voltage changes by 0.2 V as a result of that.
The line regulation = (0.2 V / 10 V) X 100 % = 2 %
Ideally, line regulation is 0%. The amount of the non-zero line regulation percentage in practical power supplies depends on system specifications, accuracy requirements and costs.