Jennica's ring is one of the rewards given after completing the Wilderness quest Spirit of Summer. It belonged to Jennica Bonde. This ring provides no stat bonuses. It is used to access portals to the parallel Spirit Realm. It is used in the sequel quest, Summer's End.
The ring will not teleport you if you have been teleblocked. To use the ring, stand directly on top of or adjacent to the portal with the ring equipped and operate it.
The ring will also double the chance of receiving a special drop from Revenants. The types of special drop depend on the type of the Revenant. For a Revenant dragon, it is a Dragon scimitar. For a Revenant Knight, it is an Amulet of glory, and so on.
If the ring is lost, it may be obtained by searching the rubble next to the quest's starting location, north of the church and museum in Varrock, on the safe side of the wilderness wall.
Also, speaking to the Strange Old Man at the entrance to the Barrows while wearing the ring allows for a few lines of dialogue regarding the ring.