The geometrical threshold sets the maximum number of pixels in USAN area around corner. If potential corner has USAN with more pixels, than it is not a corner.
Decreasing this value decreases the amount of detected corners - only sharp corners are detected. Increasing this value increases the amount of detected corners, but also increases amount of flat corners, which may be not corners at all.
Default value is set to 18, which is half of maximum amount of pixels in USAN.