The Whisper is a passive Mental Skill that works automatically with The Passage. It allows Scarlett to actually speak with those spirits of the dead that she can see with the Eye of the Lost mental skill. The presence of such restless spirits in the mortal world is usually signaled by a Skeleton lying on the ground. It lasts as long as The Passage, and has no additional cost in Mental Energy beyond what's required to invoke The Passage.
A few ghosts we'll just wish to tell Scarlett some secret before they move on to Death. Only a few of those ghost stories will be interesting (e.g. Tarka Gor.) Occasionally, they may wish to give Scarlett something from their mortal life (see the Unworldly Clutch necromantic skill.) This skill is important in getting or fulfilling certain quests (The Voice of the Dead, Lost Love, Glory Forsaken.)
The Whisper is a granted skill, bestowed on Scarlett by Benedict's ghost at the Gateway Idol in the Chapel of the Arsenal District (part of the Of Cursed Chapels quest.) Learning it doesn't require any Skill Points. It is the initial skill of the Third Twilight Circle (even though it is probably the forth Circle Scarlett opens), and learning it opens the rest of the third circle for normal learning through Necromancy Teachers and Skill Points.