Dear Aqeel,
The wind stow speed is the wind speed that causes heliostats to move into stow position to avoid being damaged. For example, if the wind stow speed is 15 m/s, then for any time step that the wind speed in the weather file is greater than 15 m/s, the heliostat will defocus and move into stow position.
Here is the definition of the wind stow speed from SAM's Help system topic for the CSP Power Tower Heliostat Field page:
Wind velocity from the weather file at which the heliostats defocus and go into stowed position. At wind speeds above the stow speed, SAM assumes that the heliostats move into stow position to protect the mirror surface and support structure from wind damage. SAM accounts for the parasitic tracking power required to stow the heliostats, and to reposition them when the wind speed falls below the stow speed.
Best regards,
Paul.