mid returns the character in theString, at the position you specify (0 for first character etc), and count = how many characters you want returned, in this case 1.
If you have a string "Hello world" and want the "w", then you'd do..
theString = "Hello world"
character = mid( theString, 6, 1 )
If you wanted "Hell" returned (who would! ;) then..
characters = mid( theString, 0, 4 )
Hope that clears it up for you.