Hello,
Tab Index works like below,
-1 = Tab is not move to this field. It will jump to the next field after this field.
0 = Tab default it goes by order. Tab will reach this field by order placed in UI.
1 = Tab is forced to move this field. If you have field tab value 1 then by default cursor will be in this field.
We can give tab order also like 1, 2, 3 .. for each field to move the tab like our specified order.