Bobby: Well you've got, just over five hours to go? You're piercing the Veil, Dean. You're glimpsing the B-side.
Dean: A little less new age-y please.Bobby: You're almost Hell's bitch. So, you can see Hell's other bitches.
The Veil appears to be the plane of existence on Earth that ghosts, hellhounds and reapers are able to inhabit. Demons and angels that are inhabiting vessels are able to pierce through and perceive the Veil, as well as interact with spirits and reapers. Symbols such Enochian runes/warding can be painted and hidden within the Veil, and can only be visible to those who can pierce/inhabit it. A human will begin to see through the Veil once a crossroads demon contract is up, allowing them to see hellhounds and the true faces of demons. Piercing the Veil can be incredibly taxing for a spirit, and takes a lot of concentration. Spirits that try to communicate can get things jumbled, only being able to get through anagrams or word fragments. The ancient Celts believed that October 31st was the one night of the year where the Veil was thinnest between the living and the dead. They used the night to appease the demon Samhain, who was able to summon spirits (among other supernatural entities) from the Veil onto the earthly plane.
It was revealed since Metatron enacted his spell, that Heaven has been locked down, forcing all spirits meant for Heaven to remain stuck in the Veil at the sites of their deaths, causing the Veil to become very crowded. The ghost of Kevin Tran described the situation as "DMV line times infinity bad," to Sam and Dean. Needing souls for their weapon to stop Amara, the reaper Billie aids Sam, Dean et all by collecting hundreds of thousands of the spirits trapped in the Veil for their "soul bomb."