Enochian sigils belong to a type of magic that originated from the enochian language. Both demons and angels have knowledge of the sigils, although the use originated from the latter. This branch of magic is heavily reliant on use of runes derived from the eponymous language of the angels. Hunters have some knowledge of it, but do not completely understand it.
There are many uses for Enochian sigils:
Sometimes referred to as "angel-proofing", these sigils can keep angels out of a building. They can be invisible to the human eye, but angels and spirits can see them. They are mainly cast by demons. This technique is first seen in Death Takes a Holiday, when Sam and Dean noticed glowing runes painted all over the funeral home. Castiel later confirms to Dean that those runes keep angels out. Crowley had Enochian sigils all over his mansion which prevented Castiel from entering (Abandon All Hope...).
Sam and Dean are later forced to write them all over Bobby's house to keep Castiel out in The Man Who Would Be King - though Bobby did not do a very good job. They get it right in The Man Who Knew Too Much.
One powerful angel ward appears in The Vessel. The ward is an enochian symbol that, if drawn on an object, even a large submarine, can protect it. It even can repel archangels. The spell was intricate and tied to the life of the caster, so even removing the sigils doesn't stop the spell.
Concealment From AngelsEdit
|“||They're Enochian sigils. They'll hide you from every angel in creation, including Lucifer." "What? You just branded us with it?!" "No, I carved in to your ribs.||”|
— Castiel and Dean
in Sympathy for the Devil
This is seen in Frontierland, when an injured Castiel paints a sigil on Bobby's fridge to hide himself while he healed. It is believed that it hides an angel's presence from any other angel.
Castiel later puts some sigils up in chalk on Rufus' Cabin to protect them from his fellow angels. These are rendered useless when Meg goes out to kill two demons and thus exposes their location, though the sigils are seen in later episodes.
Castiel, in order to hide from the angels that are after him, has an Enochian sigil tattooed on his stomach. (I'm No Angel) However, this doesn't protect him from an angel who has special abilities such as Ephraim who is able to locate him through his pain. (Heaven Can't Wait)
Demon Body ImprisonmentEdit
When sigils are carved into the human the demon is possessing, they lock the demon inside the human's body, making it unable to leave. This is first seen in The Devil You Know when Crowley carves them into Brady's chest to stop him from escaping.