I understand that you have a question about the difference equation expression in the "Dryden Wind Turbulence Model (Discrete)" block.
The difference equation in the table provides an expression that is used to update the variable for the next time step. For example, based on the current value of the longitudinal variable 'ug', its updated value can be computed from the expression.
You can also look under the mask of the "Dryden Wind Turbulence Model (Discrete)" block. In the "Filters on velocities" subsystem, you can open the "Hugw(z)" block. This subsystem realizes the expression in the table and updates value of the variable 'ug'.
From the block diagram within the "Hugw(z)" block, you can also find that 'η' is noise. The noise signal comes from a "Band-Limited White Noise" block in the "Dryden Wind Turbulence Model (Discrete)" block.
If you have any further question regarding this issue, you can contact MathWorks Technical Support for assistance.