Hayley made a big discovery about the Hollow on The Originals with a little help from her dead parents. After learning that her parents were murdered by the Hollow, Hayley decided to go looking for answers in "Bag of Cobras." With Freya's help, she was able to recover her memories as a baby when she watched her parents get murdered, and discovered that the Hollow was after a bone. The Hollow wants to be reborn on The Originals and it needs bones to do it.
In her memories, Hayley saw her parents refuse to give something to the man who killed them, mumbling something about needing all four objects before hiding a key in the floorboards. Hayley recovers the key and traces it back to a storage locker, where inside she finds a jawbone (aka a mandible) hidden inside a teddy bear. Now that they know this was what the Hollow was after all those years ago when Hayley was a baby, Elijah and Klaus realize what they're dealing with. The Hollow is a dead witch who wishes to be brought back to life, and it needs its remains to do it. The four pieces from Hayley's memory must refer to four bones, or skeleton fragments, belonging to the Hollow's original body.
According to Vincent, every time the Hollow has shown itself, it has favored four different spots around town — supposedly where those four bones were being kept. So, the question now is: where are the other three bits of remains? Unfortunately for Klaus, it seems that Papa Tunde's Blade, the dagger used to torture him by Marcel, was one of the Hollow's bones, and Dominic stole it right from under him.
The Hollow's followers have at least one of the bones they were looking for, and Hayley has another. Assuming there are still two bones out there, the fight against the Hollow just turned into a race to find two old bones.