Whats this?

Hi folks, new to fish keeping. Got 4 angelfish and now saw this. Can anyone shed some light on this thanks.

Hard to tell from the photo.

The fish looks exceptionally fat, that could be because it's full of eggs, and what you can see is the breeding tube, (ovipositor) down which the eggs are laid.
It may also be because the fish has heavily overfed, so much so that the gut has partially prolapsed.
If it hasn't overfed, then it could be suffering from bloat, usually caused by a protozoan gut infection. If so :-

1.If possible, remove the affect fish to a separate tank for treatment
2. Stop all feeding. Including in the main tank if you have moved the fish.
3. Do daily water changes/gravel cleans (in both tanks) to remove as much debris from the gravel as possible. (this is because the parasite infests in the gravel, and debris and could potentially affect your other fish.)
3. Treat (both tanks) with Octozin (3 day course)
4. Only recommence feeding once the swelling has substantially reduced.

Good luck