Leveling is the process in surveying of orienting a coordinate system to the local gravity field. Even in non-survey tasks, like house construction (or putting up a shelf!) you'll use a level to get all items lined up with gravity. In performing a photogrammetric measurement, modelling or survey you may want the results oriented to gravity as well. Once oriented to gravity you can make accurate measurements of true slope and vertical height differences.
There are a couple of ways of putting a photogrammetric project into a gravity-aligned coordinate system. A standard approach is to use external control points. Control points are points created by conventional survey methods. These XYZ points are imported into PhotoModeler using the External Geometry Explorer, and assigned to matching 2d points in the photos. PhotoModeler can then place the photogrammetric project in a coordinate system matching the control - which would typically have been leveled.