A tabindex="-1" value removes the element from the default navigation flow (i.e., a user cannot tab to it), but it allows it to receive programmatic focus, meaning focus can be set to it from a link or with scripting. This can be very useful for elements that should not be tabbed to, but that may need to have focus set to them. A good example is a modal dialog window - when opened, focus should be set to the dialog so a screen reader will begin reading and the keyboard will begin navigating within the dialog. Because the dialog (probably just a <div> element) is not focusable by default, assigning it tabindex="-1" allows focus to be set to it with scripting when it is presented.