To "null" the fringes is to adjust the pitch and roll of your sample so it is parallel to the plan of the interference fringes, which is perpendicular to the Z-scan axis.
The cartoons to the right show the impact on scan length when a flat part is nulled properly (cartoon on the right with the small green scan length outlined is nulled properly). If the differences in scan length on in a few μm to tens of μm it likely doesn't matter much. If the differences are in the 100s of μm, it will greatly affect your collection time and care should be taken to null the surface as best you can.
For a Smooth Flat Part, adjust for high contrast and the least number of fringes. On very flat parts, it is possible to null the surface such that the fringes take up the entire field of view
Below is a video showing the actual field of view (right) and a cartoon (left) depicting what is happening on a tilted smooth flat part as the instrument is moved up in Z with the joystick. Notice how the fringes travel across the field of view from upper right to lower left. The fringes indicate that not only is the sample tilted, the direction of travel describes how it's tilted. The fringe travel indicates, in this example, that the front left corner of the stage is higher.
Nulling the stage for a smooth flat part gets the sample surface and the Z-travel perpendicular, which means the surface is parallel to the fringes now. As the instrument is moved up in Z now, the fringes can be seen individually as they flash by.