When you enter data on a form, you press Tab to move from field to field, in the order they're shown in the form. The progression from field to field is the tab order. When you first create a form, the tab order runs from top-to-bottom.
When you move and rearrange fields, the tab order doesn't change automatically. For instance, if you had 10 fields arranged in a column, and you rearranged them so that the 10th one was at the beginning, the tab order would still show that field in 10th position, even though it's now at the top of the form. This makes it more difficult to fill in the form, so you'll want to adjust the tab order to reflect the new stucture of the form.
Tab Order Improvements
You may want to change the tab ...