The Other Side is a mysterious dimension where the spirits of dead supernatural beings reside. It was created by the powerful witch Qetsiyah, with the intention that should Silas die a mortal death, he would be forced to spend an eternity with her on the Other Side. It is first mentioned by Anna when she appears as a ghost and talks to Jeremy, telling him she's alone on the Other Side. Anna admits that she does not know what it is officially called. However, other residents such Sheila Bennett and Esther and every other character refer to it as "The Other Side" as well. This place is ruled by the Spirits.
So far, Jeremy has been the only one in the series consistently able to see ghosts; Matt Donovan briefly had this ability after he talked to Vicki in an in-between place, while Bonnie tried to revive him from drowning. Typically, in order to summon a ghost from the Other Side, a living person must be thinking about the person in question
Partial Veil Opening Spell
In order to drop the veil you need to charge all three points of The Expression Triangle and channel the mystical energy from Silas' tombstone. Once that happened, the veil can be dropped but only inside the expression triangle. Allowing the spirits to crossover in corporeal form within the Triangle to walk amongst the living.