Here is the default layout of the editor:
The Alignment Editor components:
For example, let’s assume that the coordinate of the first visible base of the row is N, but the row contains K gaps before the position N. The starting offset value will be N-K. The same rule is true for the ending offset.
You can turn off the Sequence offsets by unchecking the Actions ‣ View ‣ Show offsets main menu item or View ‣ Show offsets context menu item.