Display units determine the scale of your model in Autodesk Navisworks.
Display units are used to measure geometry in your scene, align appended models, set tolerances for clash detection, set texture sizes and so on.
When you open CAD and laser scan files, Autodesk Navisworks reads the units directly from the files. If this is not possible (for example, the file is unitless), Autodesk Navisworks uses the default units configured for that file type in the Options Editor whenever possible. Loaded files are scaled appropriately to the configured display units.
It is possible to rescale the file units, if they are found to be incorrect for the scene.