It is the difference between the native soil potential (pipe completely depolarized), and the polarized (off) potential. To complete a survey, you need to make sure your system is completely off to get native readings, then energize the system let it polarize, and interrupt during survey to get off readings. The idea is there is at least 100 mV of difference between these readings, so you know the pipe more electronegative than the surrounding soil.
Achieving the 100 mV criterion is no guarantee that the buried pipe is totally free of corrosion, i.e. it does not remove the need for integrity verification activities. Consult ISO 15589-1 for situations where applying this criterion is prohibited.