Basically, the amount of output shift when both inputs are equally driven (or shorted together in some measurments). In an ideal situation, only the difference between signals at the inputs will be amplified and applying the same signal to both inputs will cause complete cancellation. Equal + and - amounts added together should give zero. In practice, there are slight manufacturing differences between the two inputs which stop them showing identical performance and also in some cases bias shifting due to current injection from two sources combining. These can make the output voltage shift even if the inputs should be cancelling each other.
Brian.