Looks like typical A body door striker breakage. The part that formerly protruded out into the door jamb, for the door latch mechanism to grab onto and hold the door shut has broken the thin sheet metal of the jamb out. It screws into the back piece, which kind of slide around in that bracket on the back side, in your last pic. Being able to move around is how you adjust the striker. If you will unscrew the striker from the back piece, you will find a piece of sheet metal that perfectly matches that hole in the door jamb. You can fix it by putting a big fender washer behind the striker, and on the outside of the hole in the door jamb, and putting the square-ish piece back in the inside, and screwing it all back together. All that will make more sense when you get it unscrewed and have the 3 pieces in your hands. You can get a big fender washer at Lowes or Home Depot, on the bolt aisle. They are usually in the specialty fastener drawers. More than likely, you will need to drill the center hole of the washer out for the striker threads to go thru. Then it's trial and error getting it adjusted until you get it right. It's just a temporary fix to keep the door shut until you can get a repair patch welded in. Check your drivers side when you get that one fixed. They usually break first! Hope this helps. :)
