From The Aquarium Wiki
What is it?
The ovipositor is an organ used by some female fish for the laying of eggs. It consists of a tube which helps the fish to position the laying of eggs with some precision.
What does it look like?
This varies with species. But with the Betta splendens females it looks like a white spot just behind the ventral fins on the underside of the body when not in use.
- The Ovipositor on Wikipedia