Witnesses are characters on Sleepy Hollow. Their job is to protect the world from evil. There must be two Witnesses at a given moment so if one of them dies, the essence of their soul moves on to the next chosen Witness, suggesting that their souls are eternal. 
There must be two Witnesses active at any given point. 
As a soul of a Witness is eternal, once they die, the essence of their soul moves on to the next Witness.  For example, upon Abbie Mills's death, her soul moved on to the next chosen Witness which is someone from their extended bloodline, Molly Thomas. 
When a Witness reaches the age of 11, they are granted with the responsibility and power of a Witness. 
Like other humans, Witnesses may be made into Horsemen. The creation of a Horseman from a Witness does not trigger the calling of the next Witness. 
Only two Witnesses can exist at any one time. When Lara, a future version of Molly, traveled through time, she inherited her mantle as a Witness from her, making Molly the first Witness who had their mantle passed onto another person without dying.