After Lucifer fell from grace and rebelled, God commanded that Michael bind him into his cage deep in the pits of Hell and locked the cage with 66 Seals, which needed to be broken by any demon or angel wishing to free the Devil. As described by Castiel, the Seals are equivalent to locks on a door, and breaking them all opens the door. The Seals are metaphors, as they are events which need to occur in order to release Lucifer. Lilith, the first demon, was released by Azazel under Lucifer's orders, as she was described to have the capability to break the seals. In addition, Lilith herself was the last seal that can open the cage.
With the first seal broken, the angels head down to Earth in an attempt to stop the ascent of Lucifer, but they were foiled by the combined efforts of Lilith, and Ruby. Even though only 66 need to be broken, Anna Milton mentioned that there were over 600 possible seals - making it difficult for the angels to predict which one will be broken next. Lilith also proved to be a very capable strategist, and even managed to break at least three seals in one day. In "Lucifer Rising", Sam Winchester broke the final seal when he killed Lilith, thus setting Lucifer free. In the same episode, it is revealed that the "Senior Management" of Heaven deliberately allowed the 66 Seals to be broken so that Lucifer could be freed and then killed by Michael.
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