You can use it to sort an array of Points by their coordinates (using their x,y or z coordinates as the sorting argument),
Using your Point Array and their z-Coordinate as an argument the function will return you an Array of Points where array(0) is the Point with the smallest z coordinate and array(Ubound(array)) is the point with largest z coordinate (or the other way round)
Hope this helps and good luck with learning from another non-pro
If the high/low are aligned with the world axis and you’re not looking for the exact vertex that is the highest or lowest point, but just their Z values, then just use Rhino.BoundingBox on the mesh. That will return the 8 corner points of the world-aligned bounding box in order, the corners 0-3 will be the lower face of the box, and 4-7 the upper. You can then get individual X, Y or Z coordinates from the points. So you could extract the high/low as follows:
Dim arrBB
arrBB=Rhino.BoundingBox(mesh)
dblLowZ=arrBB(0)(2)
dblHiZ=arrBB(4)(2)
Call Rhino.AddPoint(0,0,dblLowZ)
Call Rhino.AddPoint(0,0,dblHiZ)