Firstly:
The anchor is a person who connects the real world to the “other side”. The other side is just a spell, and as long as the anchor is intact, supernatural beings are stuck in purgatory on the other side.
Either:
Kill/destroy the anchor to the other side, unveiling it and allowing all of the dead supernatural to roam the earth again.
OR
Sacrifice 12 travellers (a form of witch) in a spell to overwhelm the anchor. Everyone touches her to pass through to the other side. While she is overwhelmed, people can travel the other direction, passing back over to earth.
People who have died can return by making physical contact with the anchor while the spell is in process.
The process may be dangerous for the anchor, and not everyone that needs to can get through while the witch can keep the spell going. There is a time limit on this.
The travellers sacrificed themselves in this spell to resurrect their leader, Marcos. The 12 that died were traded for him.