It is the complete (or nearly complete) capture of everything that exists in your mind, an overall scan of everything holding your attention right now. It consists in identifying anything related to your work or your personal life which you consider should be different from how it currently is, and putting everything in one single place, a trusted container which we call the inbox. This is also known as Mental gathering.
This thought process takes place for the first time at the beginning of the implementation of the GTD methodology. Then you can do it whenever you feel like your stuff inventory is not current, with the help of this Incompletion Trigger List.