The "parenthesis" are used because it indicates (in
this case) that the prior word is a
Function called "DrawRectangle" within the "GraphicsWindow" (which was either previously defined or is using the default values (unlikely as it probably needs to be opened (permitted, defined) )).
The "i" is used so the value can be incremented (by 5) each time through the Loop (which repeats (100-1)/5 times).
Note: 100-0 would be '
better' (usually, but
not always) since then you would count 0,5,10,15,etc. instead of 1,6,11,16,etc. .
IF you actually do want to count 1,6,11,16,etc. then is is fine as it is, otherwise you want " For i = 0 To xx Step 5" (where xx is either 95 or 100).
When "GraphicsWindow.DrawRectangle" is called the values are:
GraphicsWindow.DrawRectangle(99, 99, 2, 2)
GraphicsWindow.DrawRectangle(94, 94, 12, 12)
GraphicsWindow.DrawRectangle(88, 88, 22, 22)
...
Loops often start with zero and not one so the increment would count by 5's (in this case). You would be better served by reading examples for Beginners written by a more skillful Teacher. Try a different Book.