The lateral/Y axis, or pitch axis is an imaginary line running horizontally across the sample through the center of gravity. A pitch motion is an up-or-down movement of the front or back of your sample as mounted on the stage.
The longitudinal/X axis, or roll axis is an imaginary line running horizontally though the length of the sample through its center of gravity. A roll motion is a side-to-side movement or tilting left or right of your sample as mounted on the stage.
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.
Nulling the fringes predominantly helps you with minimizing the scan length needed to measure your surface.