That's 'Expamet' (Expanded Metal) 'stop bead' or 'angle bead', which is fitted to external corners when plastering walls to get a neat and durable edge to the corners and act as a guide for the trowel. This sort of stuff:
https://www.diy.com/departments/exp...EbP7uKfQqcxHCwUClWVRoClIQQAvD_BwE&gclsrc=aw.d
Ordinarily, it's attached with the first coat of plaster, and when set, the finish plaster coat is applied. It should only come loose if it's been hit with something to dislodge it. Depending on what's behind the plasterboard you might be able to use plasterboard drywall screws (certainly not nails), to fix the bead back, but to do a decent job, given that the Expamet bead seems to have become detached for about 30cms along the wall with the plaster attached to it, I'd suggest that you need to remove that plaster and refix the bead securely, then rather than use Polyfilla, use either multi-finish plaster or Thistle finish plaster to make good the damage.
You can buy ready mixed plaster, but you can only apply about a 5mm layer, it takes about 24 hours to set and from my limited experience with it, if it's more than 5mm thick, it tends to 'slump' as it slowly dries. 1 kg of that costs about as much as a 7.5kg bag of Thistle which sets in maybe 20 minutes or so depending on how think you mix it.
These two videos might be of help and interest, albeit they go a bit beyond what you wish to do. (Note the use of PVA adhesive):
Stop bead:
Angle bead:
This is an example of 1 kg of ready mixed plaster, which as I said, takes about 24 hours to set:
https://www.diy.com/departments/dia...ixed-finishing-plaster-1kg-tub/1849576_BQ.prd
7.5kg of Thistle Finish only costs a little more, but no doubt you'd be left with 7kg surplus.
Thistle Finishing Plaster 7.5kg | Toolstation
Hope that might help a bit.
Good luck with it whatever you do.
David.