The Power Sealing Spell is a powerful binding spell to seal aspects of the power of a witch.
When Rowena MacLeod was cast out of the Grand Coven, the High Priestess Olivette cast the spell to place limitations on aspects of Rowena's magic. As a result, while Rowena could still perform very powerful magic, she considered it a "parlor trick" to what she was capable of without the spell binding her.
In 2015, Rowena sought to have the Grand Coven sanction her use of magic again by appealing to Olivette with the help of Crowley. After learning that the Grand Coven had been decimated due to the efforts of the Men of Letters, Rowena abandoned her pursuit and turned Olivette into a hamster, seeing the Grand Coven as useless to her now.
In 2018, after being resurrected for a second time after being killed by Lucifer, Rowena sought once more to break the spell in hopes that having her full powers back would protect her against Lucifer's inevitable return. Knowing that there was a ritual in the Black Grimoire that could break the spell, Rowena sought the book with the help of the Plum sisters. Though the Winchesters retained possession of the book, Sam Winchester gave Rowena the page she needed and she was able to perform the ritual to break the spell.
The exact procedure is unknown, but the spell creates magical purple bindings around the witch's limbs and neck that are usually invisible. The magical bindings have symbols on them that presumably give the binding its power.
Breaking the spell is nearly impossible for the witch under it. The Grand Coven, if they approve the witch's use of magic again, can break the spell. Another way is a ritual described in the Black Grimoire, an ancient tome of druidic magic.