It is the instrument light accessory header. It is there to allow you to plug the lights from things like tachometers into so their lights function in unison with the dash lights, including the dimmer function. If you don't have any accessories then there will be empty plugs, no worries.
I am the member formally known as RonK.
1966 Shelby G.T.350H
1966 Mustang Fastback Restomod under construction